Method for operating a vehicle with a fuel reformer

ABSTRACT

A method for operating an engine with a fuel reformer is presented. In one embodiment a method for operating an engine by injecting a gaseous fuel and a liquid fuel to at least an engine cylinder is presented. The method may prioritize the injection of the gaseous fuel in response to an amount of gaseous fuel stored in a fuel storage tank.

FIELD

The present description relates to a method for improving fuel controlof an engine. The method may be particularly useful for controlling fuelto an engine that may operate using two fuels.

BACKGROUND AND SUMMARY

Operating an engine with more than a single fuel allows an engine tooperate in a way that may improve engine operation as compared to whenonly a single fuel type is available. For example, an engine operatingwith reformed fuel may be able to tolerate a higher level of cylinderdilution than an engine operating solely with gasoline. On the otherhand, it may be desirable to operate the engine solely with gasolinewhen reformate is unavailable or in low supply. Thus, it may bedesirable to adjust operation of an engine depending on an amount ofavailable fuel. U.S. Patent Application 2008/0221778 describes a systemwherein engine speed and load are set differently when a quantity offuel stored in a second fuel tank is less than a predetermined value.

While it may make sense to adjust engine speed and load in response toan amount of fuel in a fuel tank, simply limiting engine speed and loadmay not leverage the remaining fuel in a way that improves engineoperation with the remaining fuel. Further, although it may be desirableto limit engine speed and load, such limiting may provide little benefitif the engine is operating at high dilution levels. Further still, theremay be times when it is desirable to over-ride such limiting functions.

The inventors herein have recognized the above-mentioned disadvantagesand have developed a method for improving gaseous fuel utilization. Oneembodiment of the present description includes a method for operating anengine, comprising: operating a fuel reformer and producing a gaseousfuel; and limiting a rate of injection of said gaseous fuel to at leastan engine cylinder in response to an amount of gaseous fuel in a storagetank less than a threshold amount when said storage tank is not empty.

By limiting the rate gaseous is injected to an engine in response to anamount of gaseous fuel in a storage tank less than a threshold amountwhen the storage tank is not empty, it may be possible to extend theamount of time an engine may be operated at higher dilution levels. Forexample, if an amount of fuel stored in a storage tank is less than apredetermined amount, it is possible to reduce the level of cylindercharge dilution while at the same time maintaining a level of chargedilution that is greater than if the engine is operated without gaseousfuel injection. In addition, limiting injection of gaseous fuel may beoverridden during some conditions even though the amount of storedgaseous fuel is low.

The present description may provide several advantages. Specifically,the approach may extend the range or time that an engine may operate athigher levels of cylinder charge dilution. Further, the method may allowa smaller fuel reformer to provide gaseous fuel to the engine, therebyreducing vehicle weight and cost. Further still, the method may improvevehicle emissions and fuel economy as compared to other systems that donot prioritize gaseous fuel utilization.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings,wherein:

FIG. 1 is a schematic diagram of an engine;

FIG. 2 is a flow chart for an engine dilution routine;

FIG. 3 is a flow chart for an engine knock control routine;

FIG. 4 is an example plot of a simulated engine operating map;

FIG. 5 is a simulated example plot of signals of interest operating anengine by the methods of FIGS. 2 and 3;

FIG. 6 is a flow chart for transient engine control with reformate;

FIG. 7 is a simulated example plot of signals of interest of an engineoperating with reformate by the method of FIG. 6;

FIG. 8 is a flow chart for engine air-fuel control with reformate;

FIG. 9 is an example plot of air-fuel related signals of interestoperating an engine with reformate by the method of FIG. 8;

FIG. 10 is a flow chart for a reformate prioritization; and

FIG. 11 is an example plot of simulated signals of interest when use ofreformate is prioritized by the method of FIG. 10.

DETAILED DESCRIPTION

The present description is related to operating an engine with a fuelreformer. In one embodiment, the engine may be configured with variablevalve timing and spark ignition as is illustrated in FIG. 1. The fuelreformer may allow the engine to operate with higher charge dilution(e.g., leaner or with additional EGR) and with higher density cylindermixtures. FIGS. 2 and 3 show routines of example dilution and knockcontrol routines that may be used to take advantage of reformateproduced by the fuel reformer. FIGS. 4 and 5 show example engineoperating regions and signals of interest when operating an engine withreformate. FIGS. 6-9 show example routines and engine air-fuel signalsof interest when operating an engine with reformate. FIGS. 10-11 show anexample routine and engine signals of interest when prioritizingreformate use for improving operation of an engine and conservingreformate.

Referring to FIG. 1, internal combustion engine 10, comprising aplurality of cylinders, one cylinder of which is shown in FIG. 1, iscontrolled by electronic engine controller 12. Engine 10 includescombustion chamber 30 and cylinder walls 32 with piston 36 positionedtherein and connected to crankshaft 40. Combustion chamber 30 is showncommunicating with intake manifold 44 and exhaust manifold 48 viarespective intake valve 52 and exhaust valve 54. Each intake and exhaustvalve may be operated by an intake cam 51 and an exhaust cam 53.Alternatively, one or more of the intake and exhaust valves may beoperated by an electromechanically controlled valve coil and armatureassembly. The position of intake cam 51 may be determined by intake camsensor 55. The position of exhaust cam 53 may be determined by exhaustcam sensor 57.

Intake manifold 44 is also shown coupled to the engine cylinder havingfuel injector 66 coupled thereto for delivering liquid fuel inproportion to the pulse width of signal FPW from controller 12. Fuel isdelivered to fuel injector 66 by a fuel system including fuel tank 91,fuel pump (not shown), fuel lines (not shown), and fuel rail (notshown). The engine 10 of FIG. 1 is configured such that the fuel isinjected directly into the engine cylinder, which is known to thoseskilled in the art as direct injection. Alternatively, liquid fuel maybe port injected. Fuel injector 66 is supplied operating current fromdriver 68 which responds to controller 12. In addition, intake manifold44 is shown communicating with intake plenum 42 via optional electronicthrottle 62. Throttle plate 64 controls the flow of air throughelectronic throttle 62. In one example, a low pressure direct injectionsystem may be used, where fuel pressure can be raised to approximately20-30 bar. Alternatively, a high pressure, dual stage, fuel system maybe used to generate higher fuel pressures.

Gaseous fuel may be injected to intake manifold 44 by way of fuelinjector 89. In another embodiment, gaseous fuel may be directlyinjected into cylinder 30. Gaseous fuel is supplied to fuel injector 89from storage tank 93 by way of pump 96 and check valve 82. Pump 96pressurizes gaseous fuel supplied from fuel reformer 97 in storage tank93. Alternatively, pump 96 may be omitted. Check valve 82 limits flow ofgaseous fuel from storage tank 93 to fuel reformer 97 when the output ofpump 96 is at a lower pressure than storage tank 93. Fuel reformer 97includes catalyst 72 and may further include optional electrical heater98 for reforming alcohol supplied from fuel tank 91. Fuel tank 91 may beconfigured to hold alcohol or a mixture of gasoline and alcohol. In someembodiments, alcohol may be separated from a gasoline/alcohol mixturebefore entering fuel reformer 97. Fuel reformer 97 is shown coupled tothe exhaust system downstream of catalyst 70 and exhaust manifold 48.However, fuel reformer 97 may be coupled to exhaust manifold 48 andlocated upstream of catalyst 70. Fuel reformer 97 may use exhaust heatto drive an endothermic reaction of alcohol supplied by fuel tank 91 andto promote fuel reformation (e.g., into a mixture of H₂, CH₄, and CO).

Distributorless ignition system 88 provides an ignition spark tocombustion chamber 30 via spark plug 92 in response to controller 12.Universal Exhaust Gas Oxygen (UEGO) sensor 126 is shown coupled toexhaust manifold 48 upstream of catalytic converter 70. Alternatively, atwo-state exhaust gas oxygen sensor may be substituted for UEGO sensor126.

Converter 70 can include multiple catalyst bricks, in one example. Inanother example, multiple emission control devices, each with multiplebricks, can be used. Converter 70 can be a three-way type catalyst inone example.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-onlymemory 106, random access memory 108, keep alive memory 110, and aconventional data bus. Controller 12 is shown receiving various signalsfrom sensors coupled to engine 10, in addition to those signalspreviously discussed, including: engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling sleeve 114; a position sensor134 coupled to an accelerator pedal 130 for sensing force applied byfoot 132; a measurement of engine manifold pressure (MAP) from pressuresensor 122 coupled to intake manifold 44; an engine position sensor froma Hall effect sensor 118 sensing crankshaft 40 position; a measurementof fuel reformer tank pressure from pressure sensor 85; a measurement offuel reformer tank temperature from temperature sensor 87; a measurementof air mass entering the engine from sensor 120; and a measurement ofthrottle position from sensor 58. Barometric pressure may also be sensed(sensor not shown) for processing by controller 12. In a preferredaspect of the present description, engine position sensor 118 produces apredetermined number of equally spaced pulses every revolution of thecrankshaft from which engine speed (RPM) can be determined.

In some embodiments, the engine may be coupled to an electricmotor/battery system in a hybrid vehicle. The hybrid vehicle may have aparallel configuration, series configuration, or variation orcombinations thereof.

During operation, each cylinder within engine 10 typically undergoes afour stroke cycle: the cycle includes the intake stroke, compressionstroke, expansion stroke, and exhaust stroke. During the intake stroke,generally, the exhaust valve 54 closes and intake valve 52 opens. Air isintroduced into combustion chamber 30 via intake manifold 44, and piston36 moves to the bottom of the cylinder so as to increase the volumewithin combustion chamber 30. The position at which piston 36 is nearthe bottom of the cylinder and at the end of its stroke (e.g. whencombustion chamber 30 is at its largest volume) is typically referred toby those of skill in the art as bottom dead center (BDC). During thecompression stroke, intake valve 52 and exhaust valve 54 are closed.Piston 36 moves toward the cylinder head so as to compress the airwithin combustion chamber 30. The point at which piston 36 is at the endof its stroke and closest to the cylinder head (e.g. when combustionchamber 30 is at its smallest volume) is typically referred to by thoseof skill in the art as top dead center (TDC). In a process hereinafterreferred to as injection, fuel is introduced into the combustionchamber. In a process hereinafter referred to as ignition, the injectedfuel is ignited by known ignition means such as spark plug 92, resultingin combustion. During the expansion stroke, the expanding gases pushpiston 36 back to BDC. Crankshaft 40 converts piston movement into arotational torque of the crankshaft. Finally, during the exhaust stroke,the exhaust valve 54 opens to release the combusted air-fuel mixture toexhaust manifold 48 and the piston returns to TDC. Note that the aboveis shown merely as an example, and that intake and exhaust valve openingand/or closing timings may vary, such as to provide positive or negativevalve overlap, late intake valve closing, or various other examples.

Referring to FIG. 2, a flow chart for an engine dilution control routineis shown. At 202, routine 200 determines engine operating conditions.Engine operating conditions may include but are not limited to enginespeed; engine load (e.g., engine load may be expressed at the amount ofair charge of engine cylinders divided by the theoretical maximum aircharge that a cylinder may hold at a defined pressure); ambienttemperature, pressure, and humidity; and engine torque request (e.g.,desired engine torque). Routine 200 proceeds to 204 after engineoperating conditions are determined.

At 204, routine 200 determines the available amount of reformate (e.g.,H₂, CH₄, and CO). In one embodiment, the amount of available reformatemay be determined from the temperature and pressure of a reformatestorage tank (e.g., element 93 of FIG. 1). Thus, the amount of availablereformate may be determined from the ideal gas law (e.g., pV=nRT where pis the absolute pressure of a gas, V is the volume of gas, n is thenumber of moles of the gas, R is the gas constant, and T is the gastemperature). However, the reformate tank may contain a mixture ofvaporized ethanol and reformate. Therefore, it may be desirable todetermine what fraction of gas held in the reformate tank is vaporizedethanol and what fraction of gas is reformate and then determine thepartial pressures of the gases to determine the amount of availablereformate.

In one example, the concentration of reformate stored in the reformatetank may be determined after the engine has been operated solely byinjecting liquid fuel and operating the engine under substantiallystoichiometric conditions. In particular, the amount of reformate storedin the fuel tank may be determined after the engine controller adapts toair and fuel conditions so that there is substantially no error betweenthe desired engine air amount and the actual engine air amount, and sothat there is substantially no error between the desired liquid fuelamount and the actual engine fuel amount. After errors have been adaptedout of the control system when the engine is operated solely with liquidfuel, the engine is operated under substantially the same stoichiometricconditions. Further, the cylinder air amount is increased based on thevolume of gaseous fuel injected to the engine (note that gaseousinjectors meter gas based on volume rather than mass), assuming thegaseous fuel is composed of solely reformate or some other proportion ofreformate (e.g., 50% reformate/50% vaporized ethanol). In oneembodiment, if the injected gaseous fuel is comprised solely ofreformate, the adjustments of gaseous fuel and cylinder air amountadjustment result in a substantially stoichiometric air-fuel mixturewhich is reflected in the exhaust gas oxygen concentration. During suchconditions, an adaptive fuel control multiplier has a value of 1. Notethat the adaptive multiplier is a multiplier to fuel and/or cylinder aircharge that adjusts injector timing and/or throttle position so that theair-fuel ratios of engine cylinders are substantially stoichiometric.However, if the injected gaseous fuel is comprised of vaporized ethanolor of a mixture of vaporized ethanol and reformate, the adaptive fuelcontrol multiplier may approach a value of 0.33. The adaptive fuelcontroller may approach 0.33 because the volume based stoichiometricair-fuel ratio of reformate is three times greater than that ofvaporized ethanol. As a result, the fraction of reformate may bedetermined from the adaptive fuel multiplier and interpolating between0.33 (ethanol) and 1 (reformate). Then the partial pressures of each gasand the ideal gas law may be used to determine the amount of reformatestored in the reformate tank.

In an alternative example, the method described in U.S. Pat. No.6,644,097 may be used to determine the content of the gaseous fuelmixture. After determining the amount of reformate available, routine200 proceeds to 206.

At 206, routine 200 determines the amount of fuel reformer output at thepresent operating conditions. In one example, the fuel reformer outputmay be determined from empirical data based on the temperature of thefuel reformer and the flow rate of fuel into or through the fuelreformer. In particular, a map of fuel reformer output may be stored inmemory. The map may be indexed based on fuel reformer temperature andfuel flow rate into or through the fuel reformer, and/or other operatingconditions.

In one example, the temperature of the fuel reformer may be measured bya thermocouple or thermistor. In an alternative embodiment, fuelreformer temperature may be inferred from engine temperature, enginespeed, spark timing, and engine mass flow rate. For example, empiricaldata representing fuel reformer temperature may be stored in a tableindexed with engine speed and engine mass flow rate. Data may beextracted from the table and then modified by engine temperature andengine spark timing. In one example, when engine spark is retarded fromminimum spark for best torque (MBT) or from knock limited spark, fuelreformer output of reformate may be increased in relation to the numberof angular degrees spark is retarded from MBT or knock limited spark.

The fuel flow rate to the fuel reformer may be determined from the dutycycle of a valve or injector or from the opening duration of a valve orinjector that supplies fuel to the fuel reformer. In an alternativeembodiment, the fuel flow rate to the fuel reformer may be determinedfrom a fuel flow meter. The fuel reformer output map may then be indexedby the fuel flow rate and the fuel reformer temperature to determine therate that the fuel reformer outputs gaseous fuel.

In an alternative embodiment, the gaseous fuel injector may bedeactivated and the fuel reformer tank pressure may be monitored over aperiod of time to establish the rate of pressure rise in the tank,thereby indicating the fuel reformer rate of gaseous fuel generation.The rate that reformate is produced may be determined from the rate ofpressure change in the reformate storage tank and the ideal gas law. Inparticular, V·dp/dt=dn/dt·RT may be solved for dn/dt. Further, aspreviously discussed, the partial pressures of reformate and vaporizedethanol may be determined from an adaptive fuel parameter so that theamount of pressure increase attributable to reformate may be determined.In this way, the rate of fuel reformer output may be determined Afterthe fuel reformer output rate is determined, routine 200 proceeds to208.

At 208, routine 200 judges whether the amount of available reformate isan amount that is less than a predetermined first threshold amount ofreformate. Since reformate is stored in a tank, the predetermined amountof reformate may be expressed as a mass of reformate stored in thereformate storage tank. The mass of available reformate may bedetermined based on temperature and the partial pressure of reformatestored in the reformate storage tank. In one embodiment, thepredetermined amount of reformate is a mass of reformate used to executeone or more engine operations at a desired rate of reformate use. Forexample, the predetermined amount of reformate may be expressed as themass of reformate used to accelerate a vehicle at a specified rate for aspecified amount of time when the desired amount of reformate isinjected to the engine at present engine operating conditions. Inanother example, the predetermined amount of reformate may be expressedas the mass of reformate used to cold start the engine one or more timeswhen the desired amount of reformate is injected to the engine. In stillanother example, the predetermined amount of reformate may be expressedas the mass of reformate used to operate the engine at a particular loadfor a particular amount of time when the desired amount of reformate isinjected to the engine. It should also be mentioned that thepredetermined amount of reformate may vary depending on engine operatingconditions. For example, if ambient temperature is low, thepredetermined amount of reformate may be set to a first level. Ifambient temperature is higher, the predetermined amount of reformate maybe set to a second level, the second level higher than the first level.By adjusting the predetermined amount of reformate to different levels,it is possible to conserve reformate when ambient temperature is lowwhen it may be more difficult to produce reformate.

In one embodiment, a desired amount of reformate at particular engineoperating conditions may be empirically determined based on combustionstability and engine emissions. Combustion stability and engineemissions may be related to the state of engine actuators such as EGRvalves, camshaft timing, valve lift, fuel injection timing, enginespeed, and engine load. Increasing charge dilution may increase engineefficiency and produce lower emissions because increased cylinder chargedilution may lower peak cylinder temperatures and permits the engine tooperate with less throttling. However, higher levels of cylinder chargedilution may cause combustion instability and increased engineemissions. Therefore, a gasoline fueled engine may be limited to between5% and 25% dilution to reduce the possibility of engine misfires andreduced combustion stability. Injecting reformate to engine cylindersmay improve combustion stability such that cylinder charge dilution maybe increased. Reformate increases combustion flame speed, therebyimproving combustion stability when cylinder contents are highly dilute.Thus, cylinder charge dilution may be increased when reformate iscombusted in engine cylinders as compared to when an engine is operatedsolely with gasoline or ethanol. As such, additional EGR or camshaftphasing or leaner air-fuel ratios may be tolerated by the engine withouta decrease in combustion stability when reformate is added to cylinderair-fuel mixtures.

The desired amount of reformate may be based on an amount of reformatethat provides a specific level of combustion stability and a level ofemissions. Therefore, the amount of reformate combusted by an engine mayvary depending on engine speed and load. In addition, it may be that useof additional reformate further improves combustion stability andemissions at a first engine operating condition (e.g., a first enginespeed and load); however, it may be desirable to use less reformate atthe first engine operating condition than an amount of reformate thatprovides a higher level of combustion stability during some conditionsso that fuel economy may be increased or so that reformate may beconserved for engine operating conditions where reformate may havegreater benefit (e.g., engine operating conditions where lower amountsor reformate provide greater levels of combustion stability, loweremissions, and greater fuel economy). Thus, the desired amount ofreformate at the present engine operating conditions may depend on avariety of factors. Therefore, the desired amount of reformate may beempirically determined and stored in memory of an engine controller. Inone example, the desired amount of reformate is stored in a table thatis indexed by engine speed and load.

If the available amount of reformate is less than the threshold amountof reformate at the present engine operating conditions, routine 200proceeds to 210. Otherwise, routine 200 proceeds to 220. In anotherembodiment, if the available amount of reformate is less than thethreshold amount of reformate or if the available amount of reformate isless than an amount of reformate desired to operate the engine under thepresent operating conditions for a predetermined amount of time, routine200 proceeds to 210. Otherwise, routine 200 proceeds to 220. Thus, inone embodiment, routine 200 may proceed to 210 if the available amountof reformate is less than the threshold amount of reformate and if thereis less than a predetermined amount of reformate stored in the reformatestorage tank as reserve.

At 220, routine 200 judges whether or not to conserve reformate at thepresent engine operating conditions. In one example, routine 200 mayjudge to use less reformate than is presently being produced by the fuelreformer when introducing additional reformate to the engine air-fuelmixture provides less benefit than an engine operating condition thatprovides increased benefits for adding additional reformate to thecylinder air-fuel mixture. For example, if at low to medium engine load(e.g., 0.2-0.4) and low engine speed (e.g., 700 RPM), the fuel reformeroutputs X grams of fuel and the engine consumes 0.95·X fuel to provide adesired level of combustion stability and emissions, 5% of the amount offuel produced by the fuel reformer may be added to storage rather thanmarginally improving combustion stability and emissions. The 5% may thenbe used at higher engine speeds and loads (e.g., 0.35-0.6 load and1500-3500 RPM) to improve engine emissions and fuel economy when theengine may use reformate to greater benefit.

In another example, reformate may be conserved to an amount that permitsthe engine to be restarted one or a predetermined number of times. Forexample, if the level of liquid fuel in the liquid fuel tank is below athreshold amount, the threshold amount a minimum liquid fuel amountwhere operation of the fuel reformer is allowed, then reformate storedin the reformate tank may be stored and used solely for restarting theengine. In another example, reformate may be stored until the engine isoperated above a predetermine engine load when fuel reformer operatingconditions are such that fuel reformer efficiency is low. Thus,reformate may be stored and used for selected conditions. If routinejudges to conserve reformate, routine 200 proceeds to 222. Otherwise,routine 200 proceeds to 226.

At 222, routine 200 reduces the amount of reformate injected to theengine to an amount less than the desired amount of reformate. In oneembodiment, reformate may be reduced by an empirically determinedpredetermined fraction. For example, if fuel reformer efficiency is lowat the present engine operating condition and the engine has beenoperating at the operating condition for more than a predeterminedamount of time, the amount of reformate injected to the engine may bereduced by a predetermined amount (e.g., 10%, 20%, 50%) to conservereformate. Further, the amount that reformate injected to the engine isreduced may be scalable as engine and fuel reformer operating conditionsvary. For example, operating an engine and fuel reformer at a particularset of operating conditions may initially result in a reduction of 10%in reformate injected to the engine. However, if the engine and fuelreformer continue to operate at the same conditions for more than apredetermined amount of time, the amount of reformate injected to theengine may be reduced by 20%, for example. Note that the amount ofreformate used may be reduced to zero. Routine 200 proceeds to 224 afterthe amount of reformate injected to the engine is decreased.

At 224, routine 200 adjusts actuators to account for the reduction ofreformate so that combustion stability and engine emissions may notdeteriorate more than a desired amount. Cylinder charge dilution may bereduced by adjusting camshaft timing (e.g., valve timing), valve lift,air-fuel ratio, and/or EGR valve position. Further, spark timing andfuel injection timing may also be adjusted in response to reducing theamount of reformate injected to the engine. Routine 200 proceeds to exitafter adjusting actuators.

At 226, routine 200 supplies the desired amount of reformate to theengine. Since it may be judged that there is sufficient reformate tooperate the engine as desired, reformate is injected to the engine atthe desired amount. In another example, it may be desirable under someengine operating conditions to use more reformate than the amountdesired for good combustion stability. Under these conditions it is alsopossible to increase the amount of reformate injected to the engine toan amount greater than the desired amount. For example, it may bedesirable to extract additional heat from engine exhaust during someengine operating conditions to reduce temperatures of exhaust aftertreatment devices. By increasing the use of reformate and increasing theamount of fuel delivered to the fuel reformer, it may be possible toextract additional heat from engine exhaust. In another example, theamount of gaseous fuel injected to the engine may be increased to alevel greater than the desired reformate flow rate when the amount ofreformate stored in the reformate storage tank exceeds a secondthreshold amount. Thus, the amount of reformate stored in the reformatestorage tank may be regulated by injecting additional reformate to theengine when the amount of stored reformate is greater than a secondthreshold amount.

At 228, routine 200 adjusts engine actuators to increase cylinder chargedilution up to a level where engine combustion stability and emissionsare at a desired level. As discussed at 208, the desired level of chargedilution may be empirically determined and stored in engine controllermemory. Cylinder charge dilution may be varied by adjusting camshafttiming (e.g., valve timing), valve lift, air-fuel ratio, and EGR valveposition according to engine speed, engine load, and amount or reformateinjected to the engine. Further, during some conditions the amount ofreformate consumed may be increased by retarding cylinder spark advanceso that the engine operates less efficiently, during a cold start forexample. After adjusting actuator operation to account for the amount ofreformate injected to the engine, routine 200 proceeds to exit.

At 210, routine 200 judges whether reformate production is limited bythe amount of liquid fuel in the liquid fuel storage tank. In oneembodiment, operation of the fuel reformer may be limited or stoppedwhen an amount of liquid fuel in the liquid fuel storage tank is lessthan a predetermined level. For example, if the amount of fuel stored inthe liquid fuel storage tank is less than twenty-five percent of storagecapacity, reformate output may be reduced by ten percent. And, if theamount of fuel stored in the liquid fuel storage tank is less thantwenty percent of storage capacity, fuel reformer output may be reducedby twenty-five percent. Note that fuel reformer output may be reduced tozero under some conditions. If routine 200 judges to reduce the rate ofreformate production, routine 200 proceeds to 218. Otherwise, routine200 proceeds to 212.

At 212, routine 200 judges whether or not to increase fuel reformeroutput. In one example, routine 200 may increase fuel reformer outputwhen engine efficiency is reduced by less than a predetermined amount atthe requested engine torque demand. Engine efficiency reduction may beestimated based on the amount of spark retard and by the additional fuelconsumed by operating the engine at less than MBT or knock limitedconditions. In one embodiment, engine efficiency loss may be empiricallydetermined by operating the engine on a dynamometer and adjusting sparktiming while maintaining a desired engine torque by increasing engineair flow and fuel. The empirically determined efficiency loss may bestored within engine controller memory and indexed by spark retard,engine speed, and engine load. When routine 212 judges to increase theamount of reformate produced by the fuel reformer, routine 200 proceedsto 214. Otherwise, routine 200 proceeds to 216.

At 214, engine operation is adjusted to increase fuel reformer output.In one example, routine 200 may increase fuel reformer output byincreasing the amount of liquid fuel (e.g. alcohol) delivered to thefuel reformer. The amount of liquid fuel delivered to the fuel reformermay be regulated by controlling a pump or by controlling a position of avalve. Further, fuel reformer output may be increased during someconditions by increasing engine spark retard and increasing engine airflow so as to increase the efficiency and temperature of the fuelreformer. After adjusting engine operation to increase fuel reformeroutput routine 200 proceeds to 216.

At 216, engine actuators are adjusted so that cylinder charge dilutionis at a level which provides a desired level of combustion stability andengine emissions when the desired amount of reformate is available atthe present engine speed and load. In one embodiment, the state ofactuators may be controlled according to output of one or more actuatortables that contain empirically determined states for actuators that maybe used to adjust cylinder charge dilution. The tables may be indexed bythe amount of reformate injected to the engine, the amount of availablereformate, engine speed, and engine load. Actuator state control tablesmay be provided for EGR valves, camshaft timing, air-fuel ratio, andvalve lift. After the state of engine actuators has been adjusted todilute cylinder charge to a level appropriate for the amount ofreformate available, routine 200 proceeds to exit.

At 218, engine actuators are adjusted so that cylinder charge dilutionis at a level which provides a desired level of combustion stability andengine emissions when the desired amount of reformate is available atthe present engine speed and load. Further, since the amount ofreformate may be limited in response to the amount of available liquidfuel, cylinder charge dilution may be further lowered. If the amount ofreformate injected to the engine is zero, then cylinder charge dilutionis set at a level suitable for gasoline or a mixture of gasoline andalcohol (e.g., ethanol). In one embodiment, the state of actuators maybe controlled according to output of one or more actuator tables thatcontain empirically determined states for actuators that may be used toadjust cylinder charge dilution. The tables may be indexed by the amountof available reformate, the amount of reformate injected, engine speed,and engine load. Actuator state control tables may be provided for EGRvalves, camshaft timing, air-fuel ratio, and valve lift. After the stateof engine actuators has been adjusted to dilute cylinder charge to alevel less than that supported when the desired amount of reformate isavailable, routine 200 proceeds to exit.

Thus, the method of FIG. 2 provides for operating an engine, comprising:reforming a first fuel into a gaseous fuel; operating said engine byinjecting said gaseous fuel and a second fuel to a cylinder of saidengine in response to an available amount of said gaseous fuel, enginespeed, and engine load; and adjusting an engine actuator to varycylinder charge dilution in response to said available amount of saidgaseous fuel. Further, the available amount of gaseous fuel may beincreased by retarding engine spark. Further, the gaseous fuel isdirectly injected to said engine. Further, the gaseous fuel is derivedfrom alcohol and wherein said engine actuator is one of an EGR valve, acamshaft phaser, a fuel injector, or a valve actuator. Further, theavailable amount is related to an amount of gaseous fuel stored in abuffer tank and an amount of gaseous fuel produced by a fuel reformer.Further, the reforming a first liquid fuel into a gaseous fuel isaccomplished with heat from exhaust of said engine and wherein saidgaseous fuel is comprised of H₂, CO, and CH₄. Further, the first fueland said second fuel is a same type of fuel. Further, the first fuel iscomprised of at least alcohol and wherein said second fuel is comprisedof at least gasoline.

The method of FIG. 2 also provides for a method for operating an engine,comprising: reforming a first fuel into a gaseous fuel; operating saidengine by injecting said gaseous fuel and a second fuel to a cylinder ofsaid engine in response to an available amount of said gaseous fuel,engine speed, and engine load; and increasing an amount of cylindercharge dilution as an available amount of gaseous fuel increases and asengine temperature increases. Further, the available amount of fuel islimited during a range of engine speed load conditions and wherein saidfirst fuel is reformed into a gaseous fuel comprising H₂, CO, and CH₄.Further, the available amount of fuel is limited when engine temperatureis less than a threshold. Further, the available amount of fuel islimited when a level of a fuel tank is less than a threshold level.Further, the charge dilution is increased by adjusting an actuatorcomprised from a group of an EGR valve, a valve actuator, a fuelinjector, and a cam phaser. Further, the second fuel is comprised of atleast gasoline.

The method of FIG. 2 also provides for a operating an engine,comprising: reforming a first fuel into a gaseous fuel; increasing anavailable amount of said gaseous fuel by injecting a first amount ofsaid gaseous fuel, said first amount of gaseous fuel non-zero and lessthan a desired amount of said gaseous fuel, said first amount of gaseousfuel injected to a cylinder of said engine when a first amount of saidgaseous fuel is less than a first threshold; decreasing an availableamount of said gaseous fuel by injecting a desired amount of saidgaseous fuel to said cylinder of said engine during a second condition;and adjusting an engine actuator to vary cylinder charge dilution inresponse to said first amount of said gaseous fuel or in response tosaid desired amount of gaseous fuel. Further, the first condition andsaid second condition said engine is operated at substantially a sameengine speed and load. Further, the first condition is a temperature ofsaid engine and wherein said second condition is a temperature of saidengine, said first temperature greater than said second temperature.Further, the first fuel is comprised of at least alcohol. Further, thedesired amount of said gaseous fuel is related to an engine speed and anengine load. Further, the method provides for comprising retardingengine spark to increase an amount of said gaseous fuel.

Referring now to FIG. 3, a flow chart for an engine knock controlroutine is shown. Routine 300 has several elements in common withroutine 200 of FIG. 2. In particular, 202-214 are identical to 302-314.Therefore, the description of these elements is omitted for the sake ofbrevity. Further, 220, 226, and 222 are the same as 320, 326, and 322 sothe description of these elements is also omitted for brevity.

Injecting gaseous ethanol or reformate may reduce the tendency forengine knock (e.g., auto-ignition of end gases after a spark ignitionevent) because both alcohol and reformate have higher octane numbersthan gasoline. In addition, reformate has a higher octane number thanalcohol. Therefore, cylinders may be operated under different conditions(e.g., at different pressures) depending on the type of gaseous fuelthat is injected to the engine.

In one embodiment, cylinder air charge may be increased and spark timingadvanced when gaseous alcohol is solely injected with or without liquidfuel. The amount of cylinder charge and spark advance that may betolerated by the engine for a given fuel charge fraction of alcoholbefore onset of engine knock may be empirically determined For example,an engine may be operated on a dynamometer and engine air charge may beincreased and spark advanced while the fraction of fuel chargecomprising alcohol is held substantially constant until knock ensues.The engine air amount may then be stored in memory of the enginecontroller so that cylinder air amount for a desired fuel fraction ofalcohol is known. Cylinder air amount may be adjusted by opening orclosing an air inlet throttle position, adjusting valve timing by way ofa cam phaser, adjusting valve lift, and increasing boost pressure from aturbocharger or compressor by way of a vane or waste gate indexer.Further, spark advance for a given engine or cylinder air amount, andfuel fraction comprising alcohol, may also be empirically determined andstored in memory.

Likewise, engine or cylinder air amount and spark advance may beempirically determined when at least a portion of gaseous fuel injectedto the engine is comprised of reformate. Since reformate has a higheroctane number than alcohol, the engine may be able to tolerate highercylinder charge amounts and/or more spark advance when reformate iscombusted in the cylinders. Accordingly, actuators such as the throttle,valve timing (e.g., camshaft position), valve lift, and boost pressuremay be set to positions that increase cylinder air charge when theamount of reformate injected to the engine increases. During someconditions actuators may be adjusted to increase the cylinder air amountto levels greater than when vaporized alcohol is injected to enginecylinders.

At 328, routine 300 adjusts engine actuators to decrease the possibilityof engine knock when the engine is operated with a desired amount ofreformate. If it is determined that a desired amount of reformate isavailable, then the tendency of the engine to knock may be reduced byinjecting reformate to the engine at higher engine loads. As such, theengine may be operated with higher cylinder pressures without inducingknock.

In one example, the amount of gaseous alcohol or reformate is increasedas the engine torque demand increases. Further, the amount of cylinderair charge may be increased to a level that supports the desired enginetorque. In one example, cylinder air charge may be increased byadjusting actuators as the fraction of reformate in the cylinder fuelcharge increases. Cylinder air amount may be increased by increasing athrottle opening area, increasing boost pressure, adjusting intake andexhaust valve timing, and/or adjusting valve lift. After adjustingactuator operation to account for the amount of reformate injected tothe engine, routine 300 proceeds to exit.

At 324, routine 300 adjusts actuators to limit cylinder air charge to alevel that is less than the level of cylinder air charge when thedesired amount of reformate is available. In one example, at aparticular engine speed, the engine load may be limited in response tothe amount of reformate available and to an amount of cylinder aircharge that is less than an amount that may produce a cylinder pressuregreater than a peak cylinder pressure. When less reformate than adesired amount of reformate is available to inject to engine cylinders,cylinder air charge may be limited by adjusting actuators that mayaffect the amount of cylinder air charge. For example, the throttleopening may be limited to a predetermined amount, boost pressure may belimited to a predetermined amount, spark advance may be limited to apredetermined amount, camshaft timing may be limited to a predeterminedamount, and valve lift may be limited to a predetermined amount. In oneembodiment, the cylinder air amount may be limited to an amount relatedto the available amount of reformate. In another embodiment, where thegaseous mixture is comprised of reformate and alcohol, the cylinder aircharge may be limited to a predetermined amount that is related to thefractions of alcohol and reformate comprising the gaseous fuel. Forexample, if the amount of reformate in the gaseous mixture is greaterthan the amount of alcohol, then the cylinder air charge amount may belimited to a level greater than if solely liquid fuel is injected to theengine, but less than an amount than when a desired amount of reformateis available to the engine. Thus, the amount of cylinder air chargevaries from an amount when the engine is operated by solely injectinggasoline to cylinders to an amount of air charge when the engine isoperated with a desired amount of reformate. In one embodiment, thepercent increase in cylinder air amount may be proportional to theamount of reformate injected to the engine. After adjusting actuators toa level where the cylinder air charge is reduced along with thepropensity of engine knock as compared to when the engine is operatedwith a desired amount of reformate, routine 300 proceeds to exit.

At 316, engine actuators are adjusted so that cylinder air charge thatis less than that which is available when the engine is operated with adesired amount of reformate. In one embodiment, the state of actuatorsmay be controlled according to output of one or more actuator tablesthat contain empirically determined states for actuators that may beused to adjust cylinder air amount. The tables may be indexed by theamount or percentage of desired reformate available, available reformate(e.g., the amount of reformate stored in the reformate storage tank),engine speed, and engine torque demand. Actuator state control tablesmay be provided for throttle position, camshaft timing, spark timing,and valve lift. After the state of engine actuators has been adjusted tolimit cylinder charge to a level less than that supported when thedesired amount of reformate is available, routine 200 proceeds to exit.

At 318, engine actuators are adjusted so that cylinder air charge is ata level less than that which provides all available engine torque at thepresent engine speed. Further, since the amount of reformate may belimited in response to the amount of available liquid fuel, cylindercharge may be further lowered to conserve reformate. If the amount ofreformate injected to the engine is zero, then cylinder air chargedilution is set at a level suitable for liquid injection of gasoline ora mixture of gasoline and alcohol (e.g., ethanol). In one embodiment,the state of actuators may be controlled according to output of one ormore actuator tables that contain empirically determined states foractuators that may be used to adjust cylinder air charge. The tables maybe indexed by the amount of available reformate, engine speed, andengine load. Actuator state control tables may be provided for EGRvalves, camshaft timing, and valve lift. After the state of engineactuators has been adjusted to limit cylinder air charge to a level lessthan that supported when the desired amount of reformate is available,routine 300 proceeds to exit.

Thus, the method of FIG. 3 provides for operating an engine, comprising:reforming a first fuel into a gaseous fuel; operating said engine byinjecting said gaseous fuel and a second fuel to a cylinder of saidengine in response to an available amount of said gaseous fuel, enginespeed, and engine load; and adjusting an engine actuator to varycylinder charge in response to said available amount of said gaseousfuel, said engine actuator further adjusted in response to an amount ofoctane increase provided to a cylinder air-fuel mixture by said gaseousfuel. Further, increasing the available amount of gaseous fuel byretarding engine spark and wherein the gaseous fuel is comprised of atleast H₂, CO, and CH₄. Further, the gaseous fuel is directly injected tosaid engine. Further, the gaseous fuel is derived from alcohol andwherein said engine actuator is one or more of a turbocharger wastegate, a turbocharger vane position, a compressor bypass valve, ignitioncoil output, throttle position, EGR valve, a camshaft phaser, a fuelinjector, or a valve actuator. Further, the available amount is relatedto an amount of gaseous fuel stored in a buffer tank and an amount ofgaseous fuel produced by a fuel reformer. Further, the reforming of afirst liquid fuel into a gaseous fuel comprising at least H₂, CO, andCH₄ is accomplished with heat from exhaust of said engine. Further, thefirst fuel and said second fuel are a same type of fuel. Further, thefirst fuel is comprised of at least alcohol and wherein said second fuelis comprised of at least gasoline.

The method of FIG. 3 also provides for operating an engine, comprising:reforming a first fuel into a gaseous fuel; operating said engine byinjecting said gaseous fuel and a second fuel to a cylinder of saidengine in response to an available amount of said gaseous fuel, enginespeed, and engine load; and increasing an amount of cylinder charge asan available amount of gaseous fuel increases. Further, the availableamount of fuel is limited during a range of engine speed load conditionsand wherein said gaseous fuel is comprised of at least H₂, CO, and CH₄,and wherein increasing an amount of cylinder charge as enginetemperature increases. Further, the available amount of fuel is limitedwhen engine temperature is less than a threshold. Further, the availableamount of fuel is limited when a level of a fuel tank is less than apredetermined level. Further, the charge is increased by adjusting anactuator comprised from a group of one or more of a turbocharger wastegate, a turbocharger vane position, ignition coil output, compressorbypass valve, EGR valve, a valve actuator, and a cam phaser. Further,the second fuel is comprised of at least gasoline.

The method of FIG. 3 also provides for operating an engine, comprising:reforming a first fuel into a gaseous fuel; operating said engine byinjecting said gaseous fuel and a second fuel to a cylinder of saidengine in response to an available amount of said gaseous fuel, enginespeed, and engine load; adjusting a first engine actuator to varycylinder charge dilution in response to said available amount of gaseousfuel at a first condition; and adjusting a second engine actuator tovary cylinder charge in response to said available amount of saidgaseous fuel at a second condition. Further, the first condition is afirst engine load and wherein said second condition is a second engineload. Further, the first actuator is one of an EGR valve, a cam phaser,a valve lift actuator, or a fuel injector. Further, the first fuelcomprises at least alcohol and wherein said gaseous fuel is comprised ofat least H₂, CO, and CH₄. Further, second actuator is one of anturbocharger waste gate, an EGR valve, a compressor bypass valve, aturbocharger vane actuator, cam phaser, or a valve lift actuator.Further, the spark retard may be increased to increase an amount of saidgaseous fuel.

Referring now to FIG. 4, plot 400 illustrates an example engine mapdefined by engine torque and engine speed. Areas 402-406 are shownmerely for illustration purposes and are not meant to limit the scope orbreadth of this description. The Y-axis represents engine torque andincreases from bottom to top. The X-axis represents engine speed andincreases from left to right.

Area 402 represents part load conditions when it may be desirable tooperate an engine with higher levels of charge dilution. In this area,the engine may be operated at higher dilution rates because less thanfull engine torque is requested and because the charge dilution mayimprove fuel economy and reduce engine emissions.

Area 404 represents low load engine operation when less charge dilutionmay be desirable because combustion stability may be reduced. Inaddition, when the engine is operated in this region, the engine mayproduce less heat so that the fuel reformer efficiency is reduced. Inthis region, it may be desirable to heat the fuel reformer by anelectrical heater so that reformate is available.

Area 406 represents engine operation at higher engine loads. In thisregion, it may be desirable to increase the amount of reformatecombusted in engine cylinders to control engine knock. Engine knock isproduced by the spontaneous combustion of cylinder end gases after aspark ignition has occurred. The cylinder gases may auto-ignite becauseboth cylinder temperature and pressure increase in the cylinder afterthe air-fuel mixture in the cylinder is ignited. In this engineoperating region, the fuel reformer efficiency may increase as theengine exhaust temperature increases. Higher exhaust temperatures mayimprove gasification of liquid fuels and may further increase thereformer catalyst efficiency.

Referring now to FIG. 5, is a simulated example plot of signals ofinterest when operating an engine by the methods of FIGS. 2 and 3. Thefirst plot from the top of the figure represents a desired engine torque514. The Y-axis arrow indicates a direction of increasing torque.Desired engine torque may be determined from a pedal position sensor orfrom a combination of inputs. For example, desired engine torque may bea function of a pedal position and a hybrid controller torque request.

The second plot from the top of the figure represents EGR amount 516 oranother cylinder charge dilution constituent. The Y-axis arrow indicatesa direction of increasing EGR. The EGR amount 516 may be internallysourced by adjusting valve timing or externally sourced by routingexhaust gases to the intake manifold. In an alternative example, thecharge dilution may be formed using water or excess air (lean burn).

The third plot from the top of the figure represents flow rate ofreformate to the engine. The Y-axis arrow indicates a direction of anincreasing reformate flow rate to the engine. Dotted line 518 representsthe reformate flow rate to the engine which would be desired for stablecombustion at the present operating conditions. Solid line 520represents the requested or commanded reformate flow rate to the engine.

The fourth plot from the top of the figure represents the availablereformate amount 526 in the fuel system. The Y-axis arrow indicates adirection of an increasing amount of available reformate. The availablereformate amount 526 in the fuel system may be determined from thetemperature and pressure of the reformate storage tank and from sensingthe oxygen concentration in the exhaust gases as discussed above.

The fifth plot from the top of the figure represents a cylinder chargeactuator command 528. The cylinder charge actuator may be an intakethrottle, turbocharger waste gate actuator, a cam timing actuator, valvelift actuator, or other devices that may adjust cylinder air charge. TheY-axis arrow indicates a direction of actuator movement that increasescylinder air charge. The X-axis of each of the four plots representstime and increases from the left to the right.

At time zero, indicated by the Y-axis of each plot, desired enginetorque is low and increases slightly by the time of vertical marker 500.Further, from time zero to vertical marker 500, engine EGR amount 516,cylinder charge actuator command 528, and available reformate amount 526are also at low levels. The desired reformate flow rate for stablecombustion 518 is initially higher than the commanded reformate flowrate 520 while the available amount of reformate is below a firstthreshold indicated by horizontal line 524. The first threshold level524 may vary depending on operating conditions. For example, the firstthreshold level 524 may be decreased when engine temperature is warm(e.g. engine temperatures greater than 50° C.) during a start so thatreformate may be available during idle after engine start. The firstthreshold level 524 may be lowered during while the engine is warmbecause it may be expected that the fuel reformer will have capacity toreform fuel shortly after engine start. The first threshold level 524may be raised during an engine cold start so that reformate may beconserved for engine starting. The first threshold level 524 may beraised during cold engine conditions (e.g. engine temperatures less than20° C.) because it may be expected that it will take a longer timeperiod before reformate is produced by the fuel reformer. Therefore, itmay be desirable to conserve the available reformate for higher prioritymaneuvers.

In this example, the amount of available reformate amount 526 begins lowand starts to increase before the time indicated by vertical marker 500.The rate of reformate production may increase with increasing engineexhaust temperature or by activating an electric heating element withinthe fuel reformer. In addition, pressure within the reformate storagetank may be adjusted by activating and deactivating a pump locatedbetween the fuel reformer and the reformate storage tank. In oneexample, the pump may be activated when pressure in the fuel reformerexceeds a threshold pressure.

From time zero to vertical marker 500, the cylinder charge actuatorcommand 528 is at a position which adds little or no additional air tothe base cylinder air charge amount. In one example, valve timing may beset during the illustrated conditions such that intake valve openingtiming is set to a duration that is a shorter crankshaft angle durationthan when a substantial amount of reformate is available. In anotherexample, boost pressure from a turbocharger or supercharger may bereduced by opening a waste gate or positioning a vane control.

During the time period between vertical marker 500 and 502, desiredtorque 514 continues to increase, including one step like increase. EGRamount 516 or cylinder charge dilution also increases as the engine isin a part throttle condition. Desired reformate flow rate 514 alsoincreases during this time so that engine combustion remains stable ascylinder charge dilution increases. In addition, the available reformateamount 526 also continues to increase. In one example, the availablereformate amount 526 increases as the fuel reformer temperatureincreases. Further, the cylinder charge actuator command 528 is adjustedto increase the cylinder air charge capacity. In one example, thecylinder air charge capacity may be increased as a function of availablereformate amount 526. In particular, the cylinder air charge capacitymay be increased as the available reformate amount 526 increases. Byincreasing the cylinder air charge capacity, additional air and/or EGRmay be inducted into engine cylinders. In this way, the cylinder chargecapacity may be increased as the available reformate amount increases sothat the engine may be operated at higher loads. It should also be notedthat during some conditions, the cylinder dilution may be increasedwithout increasing the cylinder charge capacity so that the engine maybe operated less throttled. During such conditions, reformate may besupplied to the engine such that the reformate improves combustionstability as compared to when no reformate is supplied to the engine atsimilar dilution levels.

During the time period between vertical marker 502 and 504, desiredtorque 514 continues to increase, including another step like increase.Further, the available reformate amount 526 increases above a secondthreshold indicated by horizontal marker 522. When available reformateamount 526 is above the second threshold, the commanded reformate flowrate 520 may be increased above the desired reformate flow rate 518 forstable combustion. During this time, the requested reformate flow rateto the engine 520 may be increased above the desired reformate flow rate518 so that the amount of liquid fuel inject to the engine may bedecreased and a substantially stoichiometric air-fuel mixture combusted.In addition, when fuel reformer output is high, additional flow from thereformate storage tank to the engine may be useful to purge gaseous fuelfrom the reformate storage tank that includes alcohol rather thatreformate. For example, if the fuel reformer is producing reformate at ahigh rate and the amount of stored reformate is greater than apredetermined amount, the rate at which gaseous fuel flows to the enginemay be increased beyond the desired rate so that the contents of thereformate storage tank are evacuated at a higher rate. In this way,reformate may be substituted for vaporized alcohol in the reformatestorage tank. It should be noted that the second available reformatethreshold 522 may be adjusted depending on operating conditions. Forexample, the second available reformate threshold amount 522 may bereduced when the rate at which reformate is produced exceeds athreshold.

During the time period between vertical marker 504 and 506, desiredtorque 514 is at the highest level in the plot. The amount of EGR amount516 or charge dilution is reduced during this period so that theadditional engine torque may be produced by the engine. In addition, thedesired reformate flow rate 518 is increased so as to reduce thepossibility of engine knock at higher cylinder loads. And, since theamount of available reformate is greater than the first threshold 524,and less than the second threshold 522, the commanded reformate flowrate 520 may be adjusted to match the desired reformate flow rate 518.If the gaseous fuel injected to the engine is comprised of a portion ofalcohol, the flow rate of gaseous fuel from the reformate storage tankto the engine may be increased to compensate for the reduction inreformate. The cylinder charge actuator command 528 is also adjusted toincrease the cylinder charge capacity. However, if reformate was notavailable or if less reformate was available, then the cylinder chargeactuator command 528 would be adjusted to a position that allows areduced cylinder charge. Thus, the cylinder charge actuator command 528may be adjusted in response to the available reformate amount 526. Forexample, as the available reformate amount 526 increases, the cylindercharge actuator command 528 may be adjusted to increase the cylindercharge capacity.

During the time period between vertical marker 506 and 508, desiredtorque 514 is reduced. In addition, the available reformate amount 526falls to a level less than the first threshold indicated by 524.Further, the requested reformate flow rate 520 from the reformatestorage tank to the engine is reduced in response to the availablereformate amount 526 being below the first threshold level. In concertwith the reduced reformate flow rate, the cylinder charge actuatorcommand 528 is adjusted to reduce the cylinder charge capacity. Duringsome conditions it may be possible to reduce the level of availablereformate stored in the reformate storage tank even when the fuelreformer is operating at full capacity because the engine may consumereformate at a rate higher than the fuel reformer produces reformate.

During the time period between vertical marker 508 and 510, desiredengine torque 514 trends lower, but available reformate amount 526increases above the first threshold 524. As a result, the requestedreformate flow rate to the engine 520 is increased as is the cylindercharge capacity, which is increased by adjusting the cylinder chargeactuator command 528. When the engine torque demand 514 goes from highload conditions to engine idle conditions, less reformate may be used tooperate the engine. As such, the available reformate amount 526 storedin the reformate storage tank may be increased.

During the time period between vertical marker 510 and 512, desiredengine torque 514 increases and the available reformate amount 526 isonce again greater than the second threshold 524. Therefore, therequested reformate flow rate to the engine 520 may be increased to alevel greater than the desired reformate flow rate to the engine 518. Inaddition, the cylinder charge actuator command 528 may be adjusted toincrease the cylinder charge capacity.

Referring now to FIG. 6, a flow chart of a method for controlling anengine during transient conditions is shown. At 602, engine operatingconditions are determined Engine operating conditions may include enginetemperature, engine intake manifold pressure, engine speed, enginethrottle position, transmission gear, as well as other engine operatingconditions.

At 604, routine 600 judges whether or not reformate is available. In oneembodiment, reformate may be judged as available when a pressure in areformate storage tank is greater than a threshold pressure. In otherembodiments, reformate may be judged available when pressure in thereformate storage tank is greater than a threshold and when the fuelreformer outputs reformate at a rate greater than a threshold rate. Instill another embodiment, reformate may be judged available when apressure in the reformate storage tank is greater than a threshold, whenthe fuel reformer outputs reformate at a rate greater than a threshold,and when an amount of liquid fuel stored in a storage tank is greaterthan a predetermined amount. If it is judged that reformate is availableroutine 600 proceeds to 608. Otherwise, routine 600 proceeds to 606.

At 606, routine 600 limits the amount of cylinder charge dilution. Bylimiting cylinder charge dilution, combustion stability may improvedduring driver tips-outs (e.g., releases an accelerator pedal) becauseless EGR is in the intake manifold and because the cylinder air amountis a higher portion of the cylinder mixture. If reformate is notavailable to inject to the engine is response to a change in acceleratorpedal position or desired engine torque, the steady state level ofcylinder charge dilution may be limited to a level that is less than ifreformate is available to inject to engine cylinders during a tip-out.In one example, cylinder charge dilution is limited to a level that doesnot result in cylinder misfire in the event of a tip-out or reduction ofdesired engine torque. Thus, the cylinder charge dilution level is setto a level that is less than an amount that results in higher engineefficiency and stable combustion. For example, if an engine cylinder iscapable of operating at a level of 30% EGR charge dilution and 24%thermal efficiency, the engine may be limited to a level of 24% EGRcharge dilution and 23% thermal efficiency. By limiting the steady statelevel of cylinder charge dilution, the engine may be capable ofwithstanding transient changes in engine torque demand without misfiringor causing drivability problems. After cylinder charge dilution islimited to a predetermined level, routine 600 exits.

At 608, the steady state cylinder charge dilution is increased ascompared to when the engine is operated without injecting reformate intothe engine. The level of dilution may be increased in response to theamount of available reformate. In particular, level of dilution isincreased in response to the rate of reformate production by the fuelreformer and the amount of reformate stored in the reformate storagetank. If the production rate of reformate is greater than a thresholdamount and the amount of reformate stored in the reformate storage tankis greater than a threshold amount, the level of dilution in enginecylinders may be increased to a level that corresponds to a dilutionlevel when a desired amount of reformate is available. If the rate ofreformate production by the fuel reformer is less than a predeterminedamount or if the amount of reformate stored in the reformate storagetank is less than a predetermined amount, then the level of dilution maybe an amount greater than when the engine is operated without reformatebut less than an amount when the engine is operated with the desiredamount of reformate. In one embodiment, the level of dilution may beadjusted based on the level of reformate stored in the reformate storagetank. Thus, if the fuel reformer is reforming fuel at a rate greaterthan the engine is using reformate, the amount of reformate stored inthe reformate storage tank may increase to a level where the amount ofreformate used by the engine is the desired amount of reformate.However, if the fuel reformer is reforming fuel at a rate that is lessthan the engine is using reformate, engine dilution may be decreaseduntil the engine is operating at a level of dilution appropriate foroperating the engine without reformate.

At 610, routine 600 judges whether or not there is a tip-out by thedriver or a reduction of the torque demand. For example, if the driveris operating the engine in a region where cylinder charge is diluted bydepressing the accelerator to a mid-position, routine 600 may judge thatthere is a tip-out when the driver releases the accelerator pedal orreduces the distance that the accelerator is depressed. In alternateembodiments, a tip-out condition may be judged when a torque request tothe engine is decreased. For example, if the vehicle is a hybridvehicle, a tip-out condition may be indicated when a hybrid controllerreduces engine torque demand when a battery becomes fully charged. Thus,a tip-out may be judged under different operating conditions and bydifferent methods. If routine 600 judges that a tip-out is present,routine 600 proceeds to 612. Otherwise, routine 600 proceeds to exit.

At 612, routine 600 increases the amount of reformate injected to theengine. The injection of additional reformate may be initiated inresponse to a change in the position of an accelerator pedal or torquedemand. In addition, the duration of adding additional reformate to theengine as well as the increase in the amount of reformate injected tothe engine may be a function of engine cylinder dilution level, enginespeed, and engine load prior to the change in accelerator position orengine torque demand. In one example, the level of increase in reformatemay be empirically determined by operating the engine on a dynamometerand determining a level of increase in reformate that provides forstable combustion and eliminates or reduces the possibility of cylindermisfire. Further, duration and amount of reformate injected to enginecylinders may be related to the number of cylinder events that it takesto reduce the level of EGR in the intake manifold. For example, if theengine is operated at 3500 RPM and 0.45 load and there is a tip-outcondition that reduces engine load to 0.2, the duration and amount ofreformate injected to the engine may be related to the number ofcylinder events that it takes to reduce the intake manifold of EGR tothe new level of EGR defined by the new or present engine speed andload. Thus, since the number of cylinder events at 3500 RPM for a givenamount of time is greater than the number of cylinder events at 2500RPM, the duration of the increase of injected reformate may be shorterat 3500 RPM than when the engine is operated at 2500. However, the rateof injecting gaseous fuel to the engine may be increased at 3500 RPM ascompared to when the engine is operated at 2500 RPM and a tip-outoccurs. After EGR in the intake manifold is reduced, the amount ofreformate injected to the engine may also be reduced. Thus, upon atip-out or reduction in engine torque, reformate may be increased andthen decreased as the engine reaches stabilized operating conditions.

Engine spark advance may also be adjusted in response to the tip-out andthe location where reformate is injected to the engine. If reformate isinjected directly to engine cylinders, spark may be held constant,advanced, or retarded at a level less than if reformate is injected tothe intake manifold upstream of engine cylinders. If reformate isinjected upstream in the intake manifold upstream of engine cylinders,spark may be initially retarded and then advanced as reformate reachesengine cylinders. As the engine reaches stabilized operating conditions,spark may be advanced or retarded to a level determined by thestabilized operating conditions. After the amount of reformate injectedto the engine is increased routine 600 exits.

Thus, the method of FIG. 6 provides for operating an engine, comprising:operating an engine by injecting a gaseous fuel and a liquid fuel to atleast an engine cylinder; diluting a mixture of at least an enginecylinder; and increasing a fraction of gaseous fuel relative to afraction of liquid fuel injected to a cylinder in response to atransient condition. Further, the gaseous fuel is comprised of vaporizedalcohol or H₂, CO, and CH₄, and wherein said transient condition is anoperator tip-out. Further, the fraction of gaseous fuel is injected toan intake manifold and wherein said mixture is diluted with one or moreof an EGR valve and camshaft phasing. Further, the fraction of gaseousfuel is injected directly into an engine cylinder. Further, the fractionof liquid fuel is greater than said fraction of gaseous fuel before saidfraction of gaseous fuel is increased. Further, the fraction of liquidfuel is less than said fraction of gaseous fuel before said fraction ofgaseous fuel is increased. Further, the spark advance is adjusted inresponse to increasing said fraction of gaseous fuel relative to saidfraction of liquid fuel injected to said cylinder.

The method of FIG. 6 also provides for operating an engine, comprising:operating an engine by injecting a gaseous fuel and a liquid fuel to atleast one engine cylinder; diluting a mixture of at least one enginecylinder with an amount of EGR; increasing a fraction of gaseous fuelrelative to a fraction of liquid fuel injected to the at least onecylinder in response to an operator tip-out; and increasing the fractionof liquid fuel relative to the fraction of gaseous fuel injected to theat least one cylinder in response to a reduction in dilution of themixture of the at least one engine cylinder. Further, the fraction ofgaseous fuel is injected directly into an engine cylinder. Further, thefraction of liquid fuel is greater than said fraction of gaseous fuelbefore said fraction of gaseous fuel is increased. Further, the fractionof liquid fuel is less than said fraction of gaseous fuel before saidfraction of gaseous fuel is increased. Further, comprising reducingdilution of the mixture of the at least one engine cylinder by reducinga gas fraction of EGR in response to said tip-out. Further, the gaseousfuel is comprised of at least H₂, CO, and CH₄. Further, comprisingincreasing said amount of EGR delivered to said at least one enginecylinder when said gaseous fuel is available, and decreasing said amountof EGR delivered to said at least one engine cylinder when said gaseousfuel is less than a threshold amount.

The method of FIG. 6 also provides for operating an engine, comprising:operating an engine by injecting a gaseous fuel and a liquid fuel to atleast one engine cylinder; and deactivating injection of said gaseousfuel in response to an increasing torque demand that exceeds athreshold. Further, increasing torque demand that exceeds a threshold isa peak torque demand. Further, reactivating injection of said gaseousfuel in response to an operator tip-out and decreasing an amount of saidliquid fuel delivered to said engine. Further, wherein EGR flow to saidat least one engine cylinder is decreased in response to said increasingtorque demand that exceeds said threshold. Further comprising increasingEGR flow to said at least one engine cylinder in response to saidreactivation of said injection of said gaseous fuel. The method furthercomprising retarding spark in response to said deactivation of injectionof said gaseous fuel.

Referring now to FIG. 7, simulated example plot of signals of interestof an engine operating with reformate by the method of FIG. 6 is shown.The first plot from the top of the figure shows throttle position 712.The throttle opening increases in the direction of the Y-axis arrow.

The second plot from the top of the figure represents EGR amount 714 oranother cylinder charge dilution constituent. The Y-axis arrow indicatesa direction of increasing EGR. The EGR may be internally sourced byadjusting valve timing or externally sourced by routing exhaust gases tothe intake manifold. In an alternative example, the charge dilution maybe formed using water.

The fourth plot from the top of the figure represents the availablereformate amount 720 in the fuel system. The Y-axis arrow indicates adirection of an increasing amount of available reformate. The availablereformate amount 720 in the fuel system may be determined from thetemperature and pressure of the reformate storage tank and from sensingthe oxygen concentration in the exhaust gases as discussed above.

The fourth plot from the top of the figure represents the reformate flowrate 722 to the engine. The Y-axis arrow indicates a direction of anincreasing rate of reformate flow to the engine. The X-axis for eachplot represents time and increases from the left to the right.

At time zero, indicated by the Y-axis of each plot, engine throttleposition is low from time zero up to vertical marker 700. In addition,from time zero to vertical marker 700, the amount of available reformate720 increases from a low amount to a level equivalent to a firstthreshold level indicated by horizontal marker 716. Engine EGR amount714 is also low during this time interval because the amount ofavailable reformate is low.

During the time between vertical markers 700 and 702, throttle position712 increases at approximately half way through the time interval. Theavailable reformate amount 720 increases above the first thresholdindicated by line 716. When the available reformate amount 720 exceedsthe first threshold level 716, injection of reformate 722 to the enginebegins. Further, engine EGR amount 714 is increased as reformate becomesavailable to the engine. Thus, by increasing reformate flow to theengine; the engine may tolerate higher levels of charge dilution whilemaintaining a desired level of combustion stability.

At vertical marker 702, the throttle position 712 is decreased in astep-like manner, known as a tip-out. During such a throttle tip-out, itis possible that the engine will begin to misfire due to high cylinderdilution. By further increasing the reformate amount injected to theengine 722, it may be possible to reduce the possibility of combustioninstability and/or engine misfire during a throttle tip-out (e.g., acommand for decreased throttle opening or a lesser torque request by wayof a pedal or other device). In one example, additional reformate 722may be injected to the engine when the rate of change in throttleposition exceeds a threshold. Further, the increase in reformate flow722 to the engine may be based on the present engine speed and load aswell as the rate of change in the throttle position 712. In particular,a control gain or multiplier that adjusts reformate flow rate 722 may bechanged in relation to the change in throttle position 712 and engineoperating conditions prior to the throttle tip-out. After the initialhigher rate of change in throttle position 712, the rate of throttlechange decreases. Accordingly, the increase in reformate flow to theengine 722 due to changing throttle position 712 is decreased. Note thatthe rate of change of a pedal sensor or other sensor may be substitutedfor the rate of change of the throttle position 712.

The change in throttle position 712 at 702 may be indicative of a lowerengine load (e.g., engine or vehicle deceleration) and a part-throttleoperating condition, whereas to the left of vertical marker 700 may beconsidered idle or near idle. After the initial change in throttleposition 712 at vertical marker 702, the throttle position 712 increasesat a medium rate. Since the engine is operating at a part throttlecondition, and since the amount of stored reformate is greater thanfirst threshold 716, the EGR amount 714 diluting cylinder charge isincreased as throttle position is opened further. By increasing cylindercharge dilution, the engine may operate more efficiently because enginepumping losses may be reduced. Thus, the reformate amount injected tothe engine 722 is adjusted in response to the change in throttleposition 712. At the same time, the amount of reformate injected to theengine 722 is adjusted in response to engine speed and load. In thisway, the flow of reformate may be adjusted to account for a change inthrottle position 712 as well as a change in engine speed and load.

Notice that injected reformate amount 722 increases even after throttleposition 712 and EGR amount 714 begin to decrease. When the injectedamount of reformate 722 is increased it becomes a higher fraction of thecylinder charge and thus promotes combustion stability. When the amountof reformate is increased in response to a tip-out, the amount of liquidfuel injected to the engine may be decreased so that the engine cylinderair-fuel ratio remains near a stoichiometric mixture. Thus, the amountof liquid fuel injected may be reduced during a tip-out in response tothe amount of reformate injected to the engine.

Between vertical markers 704 and 706, engine throttle position steadilyincreases; however, the amount of available reformate is reduced to alevel less than the first threshold amount 716. In response to a lowamount of available reformate, the engine EGR amount 714 is reduced asis the amount of injected reformate 722.

Between vertical markers 706 and 708, engine throttle position isdecreased at a low rate. In one embodiment, the rate of throttleposition decrease must be more than a predetermined amount before theamount of injected reformate 722 is increased. In this example, theamount of injected reformate 722 decreases steadily as throttle position712 decreases from vertical marker 706. During the time interval betweenvertical markers 706 and 708 the amount of available reformate 720remains below the first threshold 716. Consequently, the amount ofreformate injected 722 remains low.

Between vertical markers 708 and 710 throttle position is initiallyreduced and then increases about one third of the way between verticalmarkers 708 and 710. Further, the available amount of reformate 720increases above the first threshold 716 and the second threshold 718.Since the amount of reformate has increased above the second threshold718 the EGR amount may be further increased. However, if the amount ofEGR is at a limit, the amount of reformate injected to the engine may beincreased even though the EGR amount is at a limit.

At vertical marker 710, the throttle position decreases in a step-likemanner. In response to the decrease in throttle amount, the amount ofreformate injected to the engine 722 is increased and then decays to thesteady state desired reformate amount.

Referring now to FIG. 8, a routine for controlling an engine air-fuelratio mixture is shown. At 802, engine operating conditions aredetermined. Engine operating conditions may include but are not limitedto engine temperature, engine speed, engine load, and engine operatingtime since engine stop. After engine operating conditions are determinedroutine 800 proceeds to 804.

At 804, routine 800 judges whether one fuel is to be injected to theengine or two fuels. One fuel may be injected to the engine duringpredetermined operating conditions. In one example, a single fuel may beinjected to the engine when engine speed and load are low. For example,when engine speed is at idle speed and when engine load is less than 10%of full engine load. Two fuels may be injected to the engine at partialengine loads when dilution is limited by combustion stability, and athigher engine loads when the propensity of engine knock may beincreased. In one example where the size of the fuel reformer is reducedfor packaging the reformer in a vehicle, reformate may be stored at someoperating conditions while being consumed by the engine at other engineoperating conditions. However, for embodiments where reformate is alwaysavailable, the engine may operate using reformate all the time asreformate has a higher energy content as compared to alcohol. If routine800 judges to inject one fuel, routine 800 proceeds to 818. Otherwise,routine 800 proceeds to 806.

At 806, routine 800 ramps injection of gaseous fuel to the engine. Sincethere may be uncertainty as to the gaseous fuel composition, the gaseousfuel injection begins at a low rate defined by the minimum injectorpulsewidth. For example, the shortest electrical pulse at which theinjector provides a consistent rate of fuel delivery. The introductionof gaseous fuel is increased at a low ramp rate until the composition ofthe gaseous fuel is determined In one example, a single sensor thatsenses H₂, CO, or CH₄ may be disposed in the gaseous fuel line to theengine or in the gaseous storage tank. Since the reformer produces H₂,CO, and CH₄ in known even molar quantities a single sensor sensing anyone gas constituent may be used to determine the overall concentrationof reformate in the gaseous fuel system. For example, the amount ofreformate in the fuel system may be determined by sensing theconcentration of H₂ in the gaseous fuel system.

After the composition of fuel is determined, gaseous fuel injection maybe ramped at a second rate or gaseous fuel injection may be increased ina stepwise manner to the desired gaseous flow rate. In one embodimentthe engine controller may store data representing the composition of thegaseous fuel when the gaseous fuel is injected to the engine. The datamay then be used to set the gaseous fuel flow rate to the desired flowrate such that ramping of fuel may be shorter in duration or eliminated.In such an embodiment, the fuel composition may only be used when thefuel reformer stops adding gaseous fuel to the reformate storage tankbefore the time gaseous fuel was last injected to the engine.

At the time injection of gaseous fuel begins, the injected amount ofliquid fuel is reduced so that the engine may continue to operate at adesired air-fuel ratio, a stoichiometric air-fuel ratio for example. Inone example, where the engine is operated with alcohol as a substantialcomponent of the liquid fuel (e.g., greater than 40% alcohol), it may bedesirable to operate the engine substantially with reformate (e.g.,greater than 50% of the cylinder fuel charge fraction) and reduce theuse of liquid fuel since reformate has a higher heating value. If thegaseous fuel is comprised substantially of reformate, the amount ofliquid fuel injected to the engine may be reduced by a mass equal to themass of gaseous fuel injected Likewise, if the gaseous fuel is comprisedsubstantially of alcohol, the amount of liquid fuel injected to theengine may be reduced by a mass equal to the mass of gaseous fuelinjected to the engine; however, if the composition of the gaseous fuelis improperly assumed, the volume of fuel injected by the gaseous fuelinjector may cause the air-fuel ratio of engine cylinders to change in arich or lean direction. Accordingly, the amount of liquid fuel injectedto the engine may be reduced at a ramp rate that is a fraction of theramp rate gaseous fuel is introduced to the engine. Note that the engineneeds the same mass of reformate as it needs alcohol, whether liquid orvapor, to a given cylinder air charge. Therefore, it may be desirable toexpress the fueling in mass of injected fuel rather than by volume ofgas injected. The mass of fuel injected may be determined by inferringor detecting the gaseous fuel composition.

At 808, routine 800 monitors the exhaust gas oxygen concentration toestimate the engine air-fuel ratio. By monitoring the exhaust gas oxygenconcentration it is possible to determine the composition of the gaseousfuel. In one example as described above, an adaptive fuel parameter maybe used to estimate the composition of reformate in the gaseous fuelmixture. As a result, the amount of gaseous fuel may be increased ordecreased as the exhaust gas oxygen deviates from a desired amount. Forexample, gaseous fuel may be injected assuming that the gaseous fuelcomposition is 100% reformate. By their physics, the gaseous injectorsinject volume (i.e. moles). If the gaseous fuel is 100% reformate, theexhaust gas oxygen sensor indicates an oxygen concentration indicativeof a substantially stoichiometric air-fuel mixture. However, if thegaseous fuel is 0% reformate, the gaseous fuel injector will deliverthree times the fuel required for stoichiometric combustion. Therefore,the injection of gaseous fuel is limited until it can be ascertainedthat the fuel reformer is outputting reformate at a desired rate. If thegaseous injector is operated to inject 10% of the cylinder fuel chargein the form of reformate, but 100% vaporized alcohol is injectedinstead, then the fuel delivered will be 120% of the desired fuel andthis error state will be indicated by the exhaust gas compositionsensor(s). In this way, the gaseous fuel composition can readily beinferred. Once the fuel composition is inferred, then fuel injection cancontinue using this new composition or gaseous fuel injection may bedeactivated until the fuel reformer is outputting reformate at thedesired rate.

At 810, routine 800 judges whether or not the gaseous fuel mixture iscomprised of vaporized alcohol. In one example, routine 800 judgeswhether or not alcohol is present in the gaseous fuel in response to theamount or volume of gaseous fuel injected and the engine air-fuel ratioindicted from the exhaust gas oxygen concentration. For example, if itis judged that the engine is operating with an air-fuel mixture that isricher than anticipated if the engine were operating with gaseous fuelcomprised solely of reformate, it may be judged that alcohol is presentin the gaseous fuel mixture. If it is judged that alcohol is presentroutine 800 proceeds to 812. Otherwise, routine 800 proceeds to 816.

At 812, routine 800 determines the fuel reformer efficiency and adjuststhe gaseous fuel flow rate. When it is determined from the exhaust gasoxygen sensor and the gaseous fuel flow rate that the composition ofgaseous fuel has changed, the amount of gaseous fuel injected to theengine may be adjusted to compensate for the change in fuel composition.For example, if the fuel reformer efficiency decreases and the exhaustgas oxygen concentration is richer than expected, the amount of injectedgaseous fuel may be decreased. Further, since alcohol does not have thesame effect on charge dilution as reformate, the amount of gaseous fuelinjected during transient conditions or during periods when cylindercharge is diluted may be increased to compensate for a lower fraction ofreformate in the gaseous fuel mixture. For example, if a gaseous fuelinjected to the engine is comprised of 80% reformate, the flow rate orinjection duration of the gaseous fuel may be raised by 1/0.8. In thisway, the desired amount of reformate may be delivered to the engine evenwhen the gases stored in the reformate tank are not comprised solely ofreformate.

Fuel reformer efficiency may also be determined at 812 from an adaptiveparameter that indicates the engine would be operating at a leaner orricher mixture if fuel compensation were not provided. When injecting anamount of gaseous fuel results in an adaptive fuel multiplier of 0.33,it may be determined that the gaseous fuel is comprised of substantiallyall alcohol as the volumetric stoichiometric air-fuel ratio for alcoholis 0.33 of that of reformate. When injecting an amount of gaseous fuelresults in an adaptive fuel multiplier of 1, it may be determined thatthe gaseous fuel is comprised of substantially all reformate.Accordingly, the fuel reformer efficiency may be determined afterinserting the adaptive fuel multiplier into an equation of a straightline that describes 0.33 as 0% efficiency and 1 as 100% efficiency.

At 814, routine 800 may adjust fuel reformer output. If the fuelreformer is operating at an efficiency that is less than desired, fuelreformer output may be increased by retarding engine spark advance andincreasing the mass flow rate through the engine. For example, fuelreformer output may be increased by retarding spark and opening theengine throttle, thereby increasing exhaust heat. Further, routine 800may decrease spark retard when fuel reformer efficiency is low and afterdriver demand torque indicates that engine temperature is likely toincrease in response to the driver demand torque.

In another example, an electric heater may be activated to increase theefficiency of fuel reformer output. For example, it may be desirable toelectrically heat the fuel reformer when engine temperature is low aftera cold engine start. Further, it may be desirable to activate theelectric heater during engine deceleration or when the vehicle isdescending a hill. Thus, the electric heater may be activated whencurrent is available from the vehicle electrical system (e.g.,alternator) when little exhaust energy is available. Further, duringsome conditions it may desirable to retard engine spark and increaseengine mass flow while also activating the electric heater. Further,during some conditions it may be desirable to reduce the amount ofliquid fuel injected to into the reformer.

At 816, routine 800 adjusts actuators in response to the gaseous fuelinjected to the engine. Actuator adjustment may include and are notlimited to adjustment of spark timing circuitry, gaseous and liquidinjection timing, EGR, air-fuel ratio, and valve timing and valve liftadjustments. For example, if it is determined that a greater fraction ofalcohol is injected than expected and less reformate is injected, enginecams may be indexed such that less EGR is present in engine cylinders.Further, if the exhaust gas oxygen concentration is less than expectedthe amount of gaseous fuel injected to the engine may be decreased. Inanother example, cylinder charge may be limited or decreased byadjusting a throttle, cam, and/or compressor boost pressure. Afteractuators are adjusted routine 800 proceeds to exit.

At 818, routine 800 judges whether the injection of fuel during theprevious cylinder cycle of the cylinder scheduled to receive fuel wasinjection of a single fuel or of two fuels. If two fuels were injectedduring the last cylinder cycle of the cylinder scheduled to receivefuel, routine 800 proceeds to 820. Otherwise, routine 800 proceeds to824.

At 820, routine 800 adjusts the amount of liquid fuel injected to enginecylinders in response to the intake manifold pump down rate. Intakemanifold volume may be on the order of 1.5-2 times the enginedisplacement volume. Each time an intake valve opens a portion of theintake manifold gases may be evacuated into a cylinder. Therefore, afterinjection of gaseous fuel ceases to the engine, the intake manifoldcontents are diluted from the level of when gaseous fuel is injected tothe engine. With each cylinder that evacuates a portion of intakemanifold gases, gases in the intake manifold become increasingly dilutedwith air. Consequently, additional liquid fuel may be added to theengine at a rate that corresponds to the rate gaseous fuel is evacuatedfrom the intake manifold. In one example, the liquid fuel is increasedto the engine each time a cylinder inducts gases from the intakemanifold until a number of cylinders that evacuate the volume of theintake manifold have inducted intake manifold gases. In an alternateembodiment, the amount of liquid fuel injected increases for each enginecylinder that inducts intake manifold gases until engine cylinders haveevacuated the intake manifold. Note that these actions may beeliminated/simplified by selecting fuel injection configurations thathave little or no intake stored fuel from cylinder event to cylinderevent.

At 822, routine 800 adjusts actuators in response to the amount ofgaseous fuel remaining in the intake manifold. Actuator adjustment mayinclude and are not limited to adjustment of spark timing, gaseous andliquid injection timing, EGR, air-fuel ratio, and valve timing and valvelift adjustments. For example, if it is determined that a greaterfraction of alcohol is injected than expected and less reformate isinjected, engine cams may be indexed such that less EGR is present inengine cylinders. In addition, actuators may be advanced or retarded atdifferent rates depending on the fuel composition. For example, cams maybe indexed at a faster rate if reformate is a higher fraction of theamount of gaseous fuel injected to the engine because the cam timing maybe further advanced or retarded to provide additional internal EGR ascompared to when vaporized alcohol is the primary constituent of thegaseous fuel. Further, if the exhaust gas oxygen concentration is lessthan expected, the amount of gaseous fuel injected to the engine may bedecreased. In another example, cylinder charge may be limited ordecreased by adjusting a throttle, cam, and/or compressor boostpressure. After actuators are adjusted routine 800 proceeds to exit.

At 824, engine fueling is adjusted in response to engine torque demandand engine speed. In particular, the amount of liquid fuel injected tothe engine is adjusted in response to engine speed and engine load (ortorque demand). In one example, an engine is operated with asubstantially stochiometric air-fuel ratio. However, at engine speedswhere NO_(x) increases, the engine may be operated with a richerair-fuel mixture to reduce production of NO_(x). Conversely, when lessengine NO_(x) is produced, the engine may be operated with a leanerair-fuel mixture to reduce HC emissions. Routine 800 exits after theamount of liquid fuel injected to the engine is set to provide asubstantially stoichiometric air-fuel mixture.

Thus, the method of FIG. 8 provides for operating an engine, comprisingoperating an engine by injecting a first liquid fuel; processing asecond liquid fuel in a fuel reformer to produce a gaseous fuel; rampingin an injection amount of said gaseous fuel to said engine; andadjusting an actuator to a first state in response to said gaseous fuelwhen said gaseous fuel is a first type of gas; and adjusting saidactuator to a second state in response to said gaseous fuel when saidgaseous fuel is a second type of gas, said engine operating atsubstantially a same engine speed and load when adjusting said actuator.Further, the actuator is a fuel injector. Further, actuator is one of anEGR valve, a camshaft indexer, a fuel injector, a turbocharger, or avalve actuator. Further, the processing of said second liquid fuelincludes heating said second liquid fuel. Further, the adjusting saidactuator to said first state and said second state is in response to anoxygen sensor. Further, spark is retarded to at least a cylinder of saidengine when said oxygen sensor indicates an concentration of alcoholcombusted by said engine after ramping in said injected amount of saidgaseous fuel. Further, the method comprising estimating an efficiency ofsaid fuel reformer in response to said oxygen sensor.

The method of FIG. 8 also provides for operating an engine, comprising:injecting liquid and gaseous fuel to said engine; and adjusting anactuator to a first state when said gaseous fuel is comprised of a firstgas; and adjusting said actuator to a second state when said gaseousfuel is comprised of a second gas, said engine operating atsubstantially a same speed and load when adjusting said actuator to saidfirst and second positions. Further, the gaseous fuel is produced by areformer heated by exhaust from said engine. Further, the first gas iscomprised of vaporized ethanol and wherein said second gas is comprisedof vaporized H₂, CO, and CH₄. Further, the actuator is an ignitiontiming circuit, and wherein spark is retarded as concentration ofethanol in said gaseous fuel increases.

A method for operating an engine, comprising: injecting liquid gaseousfuel to said engine; ramping in injection of a gaseous fuel to a numberof cylinders of said engine; ramping out injection of said liquid fuelat a first rate when said gaseous fuel is comprised of a first gas; andramping out injection of said liquid fuel at a second rate when saidgaseous fuel is comprised of a second gas. Further, the second rate isless than said first rate and wherein said first gas is comprised ofreformate and wherein said second gas is comprised of alcohol. Further,the number of cylinders is less than the total number of enginecylinders. Further, the number of cylinders is one cylinder. Further,the method comprising advancing spark as said gaseous fuel is ramped in.Further, the spark is advanced at a first rate when said gaseous fuel iscomprised of a first gas, and wherein said spark is advanced at a secondrate when said gaseous fuel is comprised of a second gas. Further, theadjusting said actuator to said first state and said second state is inresponse to an oxygen sensor. Further, wherein spark is retarded to atleast a cylinder of said engine when said oxygen sensor indicates anconcentration of alcohol combusted by said engine after ramping in saidinjected amount of said gaseous fuel. Further, wherein a position of athrottle plate is adjusted in response to said retarding of said spark.

Turning now to FIG. 9, a simulated plot of air-fuel related signals ofinterest operating an engine with reformate is shown. The first plotfrom the top of the figure represents the injection of one or two fuels.In particular, when signal 900 is low (e.g. at the level of the X-axis),fuel injection is by way of a single fuel injector at each enginecylinder that receives injected liquid fuel. When signal 900 is high(e.g. at the level of 2 of the Y-axis), fuel injection is by way of twofuel injectors for each engine cylinder. In particular, gaseous fuel isinjected by a gaseous fuel injector and liquid fuel is injected by asecond injector.

The second plot from the top of the figure represents flow of gaseousfuel to the engine. Gaseous fuel may be comprised of reformate or of acombination of reformate and vaporized alcohol. If the gaseous fuel iscomprised of substantially all reformate, a given volume of the gaseousfuel will produce an air-fuel mixture that is leaner than if the samevolume of vaporized alcohol is introduced to the engine under similarconditions. As discussed above, reformate has a volume basedstoichiometric air-fuel ratio that is three times greater than that ofvaporized ethanol. Thus, it may be desirable to know the composition offuel prior to injection. However, the composition of gaseous fuel maychange unexpectedly at times. For example, if the efficiency of the fuelreformer increases or decreases, the composition of gaseous fuel storedin the reformate fuel tank may likewise change. Therefore, it may bedesirable to ascertain the gaseous fuel composition before a largeamount of gaseous fuel is injected to the engine.

The third plot from the top of the figure represents the oxygenconcentration remaining in engine exhaust gases after combustion of airand fuel in engine cylinders. Horizontal line 914 represents an exhaustoxygen concentration for a stoichiometric air-fuel mixture aftercombustion. When exhaust gas oxygen trace 916 is above line 918, theengine air-fuel mixture is lean. When exhaust gas oxygen trace 916 isunder line 918, the engine air-fuel mixture is rich.

The fourth plot form the top of the figure represents the amount ofliquid fuel injected to the engine 920. The amount of liquid fuelinjected to the engine 920 is related to the amount of air in thecylinder charge as well as the amount of gaseous fuel injected to theengine. Further, since the volume based stoichiometric air-fuel ratio ofreformate is three times greater than that of vaporized ethanol, theamount of liquid fuel injected 920 may be adjusted in response to thecomposition of the gaseous fuel.

The fifth plot from the top of the figure represents spark retard fromdesired spark timing 922. For example, if gaseous fuel is injected to anengine and the desired gaseous fuel is reformate, the engine may be ableto tolerate more spark advance than if the injected gaseous fuel isvaporized alcohol. Thus, if vaporized alcohol is injected to the enginerather than reformate, spark may be retarded because vaporized alcoholraises the octane of fuel injected to the cylinder less than reformate.Alternatively, a cylinder charge actuator (e.g., camshaft) may beadjusted as a function of the amount of reformate and/or alcoholdetected in the gaseous fuel mixture.

At time zero, indicated by the Y-axis of each plot, a single fuel isinjected to the engine (e.g., alcohol, gasoline, or a mixture ofgasoline and alcohol) as indicated by trace 900 up to the timeillustrated by vertical marker 902. During the time period between timezero and vertical marker 902, the engine exhaust exhibits an oxygenlevel that indicates substantially stoichiometric combustion.

At the time of vertical marker 902, gaseous fuel is injected to theengine. Simultaneously, the amount of liquid fuel injected to enginecylinders is reduced in expectation that the gaseous fuel will richenthe cylinder air-fuel mixture. In one example, the gaseous fuel isinitially estimated to be comprised of substantially all reformate. Inanother example, the gaseous fuel is initially estimated to be comprisedof substantially all vaporized alcohol. In yet another example, aportion of the gaseous fuel is estimated to be comprised of reformatewhile the remaining portion of gaseous fuel is estimated as vaporizedalcohol. Further, one or more memory locations in the engine controllermay be used to make an initial estimate of the gaseous fuel composition.In particular, the memory locations are configured to contain anestimate of the fraction of reformate in the gaseous fuel. The estimateis based on the volume of gaseous fuel injected to the engine andexhaust gas oxygen concentration feedback from an oxygen sensor asdescribed above. Further, the gaseous fuel composition is based on theinjection of gaseous fuel during an engine operating interval duringwhich gaseous fuel was injected to the engine, the operating intervalbefore the present interval of gaseous injection and before an intervalwhen liquid fuel was solely injected to fuel the engine.

As shown in the second plot from the top of FIG. 9, the amount ofgaseous fuel injected to the engine is ramped in so that the exhaust gasoxygen concentration changes slowly rather than in a stepwise manner. Byramping in injection of gaseous fuel, the possibility of largeexcursions in exhaust gas concentration may be reduced. For example, asshow in FIG. 9, the engine exhaust gas concentration starts to go lean(e.g., above stoichiometric indicator line 918) indicating that theinjection of liquid fuel 920, based on the estimated gaseous fuelcomposition, has been reduced too much. Therefore, the engine controllerrecognizes the lean condition in response to the oxygen sensor outputand then increases the amount of liquid fuel injected. Alternatively,the engine controller can increase the amount of gaseous fuel injectedto the engine if desired.

At the time of vertical marker 904, the injection of gaseous fuel isceased and the injection of liquid fuel increases to a level necessaryfor stoichiometric combustion. In one example, where gaseous fuel isinjected to the intake manifold, it takes a number of cylinder intakeevents to evacuate the gaseous fuel from the intake manifold. Therefore,the amount of liquid fuel injected to the engine is increased at a raterelated to the evacuation of gaseous fuel from the intake manifold asmay be solved by a pump-tank differential equation.

At the time of vertical marker 906, gaseous fuel is again injected toengine cylinders at a desired rate based on engine operating conditions.For example, at a particular engine speed and load, a specific gaseousfuel flow rate may be injected to the engine. The amount of liquid fuelinjected to the engine may be reduced based on the gaseous fuel flowrate and the gaseous fuel composition. For example, if the gaseous fuelis comprised of a higher concentration of reformate, the amount ofliquid fuel injected to the engine may be reduced more/less than if thegaseous fuel is comprised of a higher concentration of vaporizedalcohol. In addition, as indicated by the fifth plot from the top of thefigure, cylinder spark may be retarded from an amount that would bedesirable if gaseous fuel were comprised of substantially all reformate.In this way, spark advance may be controlled as a function of theconcentration of reformate or alcohol in the gaseous fuel.

At the time between vertical marker 908 and vertical marker 910, asingle liquid fuel is injected to the engine cylinders. Again, theamount of liquid fuel injected may increase at a rate related toevacuation of gaseous fuel from the intake manifold. In addition, sparkis returned to a spark advance desired when only liquid fuel is injectedto engine cylinders at the present engine operating conditions.

At the time between vertical marker 910 and 912, gaseous fuel is againinjected to the engine and the amount of liquid fuel injected to enginecylinders is again reduced. The injection of liquid fuel may be reducedat a rate that is related to the rate of filling of the intake manifoldwith gaseous fuel. After vertical marker 912, injection of the gaseousfuel is stopped and the amount of liquid fuel injected to the engine isincreased.

In an alternate example, the gaseous flow rate is set so that thedesired amount of reformate is injected to the engine. For example, ifthe gaseous fuel is comprised of 80% reformate and 20% vaporizedalcohol, the amount of gaseous fuel injected may be regulated to thedesired reformate flow rate divided by 0.8. Thus, the overall gaseousflow rate is increased so that the desired amount of reformate isdelivered to the engine. The amount of liquid fuel injected to theengine between marker 906 and marker 908 is reduced based on thecomposition of the gaseous fuel and the gaseous fuel flow rate. As aresult, the engine exhaust gases exhibit an oxygen concentration that isindicative of a substantially stoichiometric air-fuel mixture.

Referring now to FIG. 10, flow chart for a reformate prioritization isshown. At 1002, routine 1000 determines engine operating conditions.Engine operating conditions may include but are not limited to enginespeed; engine load (e.g., engine load may be expressed at the amount ofair charge of engine cylinders divided by the theoretical maximum aircharge that a cylinder may hold at a defined pressure); ambienttemperature, pressure, and humidity; and engine torque request (e.g.,desired engine torque). Routine 1000 proceeds to 1004 after engineoperating conditions are determined.

At 1004, routine 1000 determines the available amount of reformate, thedesired rate of reformate consumption for stable combustion, and firstand second threshold amount of reformate at the present or currentengine operating conditions. The available amount of reformate may bedetermined as described for 204 of FIG. 2. The desired amount ofreformate may be determined as described for 208 of FIG. 2.

The first and second threshold amounts of reformate consumption may bedescribed as an amount of reformate used to carry out a number ofpredefined engine operations at present engine operating conditions. Forexample, the first threshold amount of reformate may be an amount ofreformate to accelerate the vehicle at base conditions (e.g., no grade,loaded with two passengers and no cargo) from the present speed toanother predefined speed within a predetermined time period. The secondthreshold amount of reformate may be the first amount of reformateincreased by a factor of three so as to increase an available number ofmaneuvers that can be performed with stored gaseous fuel.

In another example, the first threshold amount of reformate may be anamount of reformate used to operate the engine at a vehicle speed with alevel of cylinder dilution that provides a predetermined rate of fuelconsumption for a predetermined amount of time. The second thresholdamount in this example may be the first threshold amount increased by apredetermined factor, four for example.

In still another example, the first threshold amount of reformate may bethe amount of reformate that is produced by the fuel reformer over adefined time period (e.g., 10 minutes). In still another example, thefirst threshold amount of reformate consumption may be the amount ofreformate that is produced by the fuel reformer over a predefined timeperiod (e.g., two minutes) plus an additional amount of reformate. Forexample, if the reformate storage tank is capable of storing 100 cm³ ofgaseous fuel at a pressure of 10 bar. The first threshold amount ofreformate may be 100 cm³ of reformate at a pressure of 4 bar. In thisexample, the second threshold amount of reformate may be 100 cm³ ofreformate at a pressure of 8 bar.

In addition, the first and second threshold amounts of reformate may beadjusted. For example, the first and second threshold amounts may beadjusted for engine operating conditions, fuel reformer operatingconditions, desired engine fuel consumption rate, and atmosphericconditions.

In another example, the first threshold amount of reformate may be theamount of reformate that can be used to start the engine and heat up theexhaust after treatment system to a desired temperature.

In still another example, the first threshold amount of gaseous fuelstored in the reformate storage tank may be fixed at a constant value.In still another example, the first and second threshold amounts ofgaseous fuel stored in the reformate storage tank may be a function ofthe amount of reformate in the stored gaseous fuel. For example, if thegaseous fuel is comprised of substantially reformate the first thresholdamount of gaseous fuel may be set to a first amount. If the gaseous fuelis comprised of reformate and vaporized alcohol, the first thresholdamount of gaseous fuel may be set to a second amount, the second amountgreater than the first amount. The second threshold amount of reformatemay be adjusted in a similar manner. In this way, the first and secondthreshold amounts of gaseous fuel may be adjusted so that the amount ofreformate stored in the reformate tank is substantially the same eventhough the gaseous fuel includes vaporized alcohol. Routine 1000proceeds to 1006 after determining the available amount of reformate,desired rate of reformate consumption, and the first and secondthreshold amounts of reformate.

At 1006, routine 1000 judges whether or not an amount of gaseous fuelstored in the reformate storage tank is less than the first threshold.If routine 1000 judges an amount of stored reformate is less than afirst threshold, routine 1000 proceeds to 1014. Otherwise, routine 1000proceeds to 1008.

At 1008, routine 1000 judges whether or not the amount of gaseous fuelstored in the reformate storage tank is greater than a second threshold.In one example, the second threshold amount of gaseous fuel stored inthe reformate storage tank may be an amount greater than the firstthreshold by predetermined factor (e.g., 3 times the first thresholdamount). In another example, the second threshold amount of gaseous fuelstored in the reformate storage tank may vary with operating conditions.For example, the second threshold amount of gaseous fuel stored in thereformate storage tank may be reduced to 65% of the storage tankcapacity when the fuel reformer is producing gaseous fuel at a rategreater than a predetermined amount. In still another example, thesecond threshold amount of reformate or gaseous fuel stored in thereformate storage tank may be a related to the desired rate of reformateuse. For example, if the desired rate of reformate use is a firstamount, the second threshold amount of gaseous fuel stored in thereformate storage tank may be a first amount. If the desired rate ofreformate is a second amount, greater than the first desired rate ofreformate use, the second threshold amount of gaseous fuel stored in thereformate storage tank may be increased to a second amount, greater thanthe first amount when the desired rate of reformate use is a firstamount.

If routine 1000 judges an amount of stored reformate is greater than thesecond threshold amount of reformate, routine 1000 proceeds to 1010.Otherwise, routine 1000 proceeds to 1018.

At 1010, routine 1000 increases the rate of gaseous fuel consumptionabove the rate of reformate consumption. In one example, when thegaseous fuel is comprised of reformate and vaporized alcohol, the amountof gaseous fuel injected to the engine is increased to the engine to alevel such that the amount of reformate contained in the gaseous fuelinjected to the engine is substantially equal to the desired rate ofreformate use. In another example where the gaseous fuel is comprised ofsubstantially all vaporized alcohol, the rate of injection of gaseousfuel may be increased above the desired rate of reformate use so thatthe vaporized alcohol can be evacuated from the reformate storage tankand replaced with reformate. Routine 1000 proceeds to 1012 afteradjusting the reformate use.

At 1012, routine 1000 adjusts injection of liquid fuel in response toincreasing or decreasing injection of gaseous fuel. When the amount ofgaseous fuel injected to an engine is changed, and the new amount ofgaseous fuel comprises a lesser or greater portion of the cylinder aircharge, the amount of injected liquid fuel may be adjusted to compensatefor the change in gaseous fuel. In one example, the amount of liquidfuel reduction may be a function of the gaseous fuel composition and thevolume of gaseous fuel injected. For example, if the gaseous fuel iscomprised substantially of alcohol and injected at a first volumetricflow rate, the amount of liquid fuel injected to the engine may bereduced by a first amount. On the other hand, if the gaseous fuel iscomprised substantially of reformate and injected at the firstvolumetric flow rate, the amount of liquid fuel injected to the enginemay be reduced by a second amount, less than the first amount.

In another example when the amount of gaseous fuel injected isdecreased, the amount of liquid fuel injected to the engine may beincreased at a rate that corresponds to a rate at which the intakemanifold is evacuated of gaseous fuel. Further, the amount of increasein liquid fuel injection amount may be related to the fuel type andconcentration of fuel type in the gaseous fuel mixture. For example, ifthe gaseous fuel is comprised of solely reformate, the amount of liquidfuel may be increased by a first amount for each cylinder intake eventat a particular engine operating condition. On the other hand, if thegaseous fuel is comprised of solely vaporized alcohol, the amount ofliquid fuel may be increase by a second amount for each cylinder intakeevent during similar operating conditions.

At 1014, routine 1000 judges whether or not to increase the rate ofreformate production. In one example, routine 1000 requests additionalreformate when the rate of reformate production is less than a thresholdrate. The threshold rate may vary with operating conditions. Forexample, shortly after an engine cold start, the threshold rate ofreformate production may be a first amount. After engine temperatureincreases the threshold rate of reformate production may be increased toa second amount. If routine 1000 judges it desirable to increasereformate production, routine 1000 proceeds to 1020. Otherwise, routine1000 proceeds to 1016.

At 1020, routine 1000 adjusts engine operation to increase reformategeneration. In one example, the rate of reformate production may beincreased when engine exhaust temperature is greater than a threshold byincreasing fuel flow to the fuel reformer. In this way, the amount ofexhaust heat transferred to the fuel reformer may be increased to reduceengine exhaust temperature and to protect exhaust after treatmentdevices. In another example, engine spark retard may be increased toincrease fuel reformer output so that additional reformate is availableto the engine during engine conditions where exhaust gas temperaturesmay be low. For example, after an engine cold start or during anextended engine idle period (e.g., idle greater than 2 minutes), enginespark may be retarded and the intake throttle may be opened to increaseexhaust gas temperature and the gas flow. The additional exhaust gasheat and mass flow may be used to increase the output of the fuelreformer.

During engine accelerations or during higher engine loads, spark may beadjusted from retarded conditions to minimum spark for best torque (MBT)or to knock limited spark to increase engine efficiency. Engine exhaustgas temperatures increase when an engine is operated at higher loadsbecause the cylinder charge is increased which allows the cylindertemperatures and pressures to increase. Therefore, during suchconditions, there may be sufficient exhaust gas energy to operate thefuel reformer at rated capacity without spark retard. Thus, spark retardmay not be required or may be reduced during such conditions.

Fuel reformer output may also be increased by activating an electricheater within the fuel reformer when a temperature of the fuel reformeris less than a threshold temperature. In one example, an electric heaterin a fuel reformer may be activated during a cold engine start toincrease engine load so that exhaust gas temperatures increase at anaccelerated rate. During a cold engine start, a substantial amount ofexhaust gas energy is imparted to exhaust after treatment devices (e.g.,catalysts) which may leave little heat to activate the fuel reformer.During such conditions, it may be desirable to activate an electricheater so that reformate may be produced before engine exhaust gases arehigh enough to activate the fuel reformer. After the engine is up tooperating temperature the electric heater may be deactivated. Thus,during a first condition, the primary heat source for the fuel reformermay originate from an electric heater. And, during a second condition,the primary heat source for the fuel reformer may originate from theengine. Routine 1000 proceeds to 1016 after adjusting the engine toincrease reformate production.

At 1016, routine 1000 judges if the desired rate of reformateconsumption is greater than a threshold rate of reformate consumption.The threshold rate of reformate consumption may be a function ofdifferent operating conditions and may be a different rate of gaseousfuel injection for different engine operating conditions. For example,the threshold rate of reformate consumption may be an amount of gaseousfuel injected at a minimum pulse width of the gaseous fuel injector.Alternatively, the threshold rate of reformate consumption may be areformate flow rate that provides a desired level of emissions and/orfuel economy at a particular engine operating condition. In anotherexample, the threshold rate of reformate consumption may be the rate atwhich reformate is presently being produced by the fuel reformer. Instill another example, the threshold rate of reformate consumption maybe the highest rate that reformate may be produced by the fuel reformer.If the desired rate of reformate consumption is greater than thethreshold rate of reformate consumption by the engine, routine 1000proceeds to 1022. Otherwise, routine 1000 proceeds to 1018.

At 1018, reformate is consumed or used by the engine at the desiredrate. Reformate may be injected to the engine at the desired rate byadjusting the position or timing of an injection valve and by accountingfor the pressure in the reformate storage tank. If the pressure in thereformate tank increases, the gaseous fuel injector may be operated at alower duty cycle so as to shorten the gaseous fuel injection time andcompensate for the increase in reformate tank pressure. Similarly, ifthe pressure in the reformate tank decreases, the gaseous fuel injectormay be operated at a higher duty cycle so as to lengthen the gaseousfuel injection time and compensate for the decrease in reformate tankpressure. If the pressure drop across the fuel injector is sufficient toestablish sonic flow, the injector timing may not be adjusted since thegaseous flow rate will remain substantially the same.

At 1022, routine 1000 may limit the amount of reformate injection.Limiting the injection of reformate may include eliminating injection ofreformate to the engine. Further, during some conditions the limiting ofreformate injection may be overridden such that the desired amount ofreformate is injected to the engine even though the amount of gaseousfuel stored is less than the first threshold amount. For example,limiting of injection gaseous fuel may be overridden when a torquerequest by an operator exceeds a predetermined threshold. Alternatively,limiting of injection of gaseous fuel may be overridden when the desiredrate of gaseous fuel consumption is less than the rate reformate isbeing produced by the fuel reformer.

On the other hand, reformate injection may be limited when reformatestored in the reformate storage tank is less than the first thresholdamount of reformate as determined at 1004. In another example, reformaterate of injection may be limited to a fixed value that is expected to beless than the rate of reformate production by the fuel reformer. Inanother example, reformate injection rate may be limited to a value thatvaries with engine operating conditions. For example, at a firstoperating condition, if the desired rate of reformate consumption isgreater than a threshold, the amount of reformate injected may be afirst fraction or percentage of the desired rate of consumption. At asecond operating condition, if the desired rate of reformate consumptionis greater than a threshold, the amount of reformate injected may be asecond fraction or percentage of the desired rate of consumption. In oneexample, the first and second fractions may be based on a priority ofoperating conditions. For example, it may be more desirable to limitgaseous fuel injection to a higher percentage of the desired rate ofreformate consumption when the engine is under higher loads (e.g.,during a throttle tip-in or during acceleration) as compared to when theengine is operated at lower steady state conditions (e.g., highlydiluted part-throttle conditions). Routine 1000 proceeds to 1012 aspreviously described, then proceeds to exit.

Referring now to FIG. 11, an example plot of simulated signals ofinterest when use of reformate is prioritized is shown. The first plotfrom the top of the figure represents the available amount of reformatein the fuel system. The Y-axis arrow indicates a direction of anincreasing amount of available reformate. The amount of reformate in thefuel system 1120 may be determined from the temperature and pressure ofthe reformate storage tank and from sensing the oxygen concentration inthe exhaust gases as discussed above. Horizontal marker 1118 representsa first threshold amount of available reformate. Horizontal marker 1116represents a second threshold amount of available reformate.

The second plot from the top of the figure represents the amount ofgaseous fuel injected to the engine. The amount of gaseous fuel injectedto the engine 1122 increases in the direction of the Y-axis arrow.

The third plot from the top of the figure represents the amount ofliquid fuel injected to the engine. The amount of liquid fuel injectedto the engine 1124 increases in the direction of the Y-axis arrow.

The fourth plot from the top of the figure represents spark retard fromdesired spark timing. The amount of spark retard 1126 increases in thedirection of the Y-axis arrow.

At time zero, indicated by the Y-axis of each plot, to the time ofvertical marker 1100, the amount of available reformate 1120 is at a lowlevel but gradually increases. During this time period, the amount ofavailable reformate 1120 is less than the first threshold level 1118. Inthe same time period, gaseous fuel injection 1122 is deactivated whileliquid fuel injection 1124 is at a higher level and combusted with airin a substantially stoichiometric mixture. In addition, the engine spark1126 is initially retarded. In this example, engine spark is retarded toincrease exhaust gas temperature so that output of the fuel reformerincreases. The fuel reformer efficiency may be increased by increasingthe temperature of the fuel reformer, at least up to a thresholdtemperature.

At time marker 1100, the amount of available reformate 1120 increases toan amount greater than the first threshold 1118. At the same time,injection of gaseous fuel 1122 to the engine begins and injection ofliquid fuel to the engine is reduced. Further, since the amount ofavailable reformate 1120 is greater than the first threshold 1118, thespark retard 1126 is reduced to increase engine operating efficiency.

At time marker 1102, the amount of available reformate 1120 exceeds asecond threshold level 1116. When the available amount of reformate 1120is greater than the second threshold level, the engine controller allowsthe amount of gaseous fuel injected to the engine 1122 to increase to alevel that is greater than the desired level of gaseous fuel flow. As aresult, injection of gaseous fuel 1122 is increased at marker 1102.Combustion of a stoichiometric mixture in engine cylinders continues atvertical marker 1102 by reducing the amount of liquid fuel injected 1124in relation to the amount of gaseous fuel injected. In this example,spark retard 1126 remains low between vertical marker 1102 and 1104.However, in other examples, spark may be retarded when a portion ofgaseous fuel injected is alcohol rather than reformate.

At time marker 1104, the amount of available reformate 1120 falls belowthe second threshold 1116. The engine controller reduces the flow ofgaseous fuel 1122 to the engine in response to the available amount ofreformate being less than the second threshold. Accordingly, the amountof liquid fuel injected to engine cylinders 1124 is increased so thatstoichiometric combustion can continue. Spark retard 1126 remains lowuntil the time of vertical marker 1106.

At vertical marker 1106, the amount of available reformate 1120decreases to a level less than the first threshold level 1118. Theengine controller deactivates gaseous fuel injection 1122 at verticalmarker 1106 to conserve reformate for higher priority engine operatingconditions (e.g., engine cold starting or high torque demandconditions). Again, the amount of liquid fuel injected to enginecylinders 1124 is increased so that stoichiometric combustion continues.Further, engine spark retard 1126 is increased so that fuel reformerefficiency can be increased. Engine spark retard may reduce enginetorque, so engine air charge may be increased while spark is retarded sothat driver demand torque is maintained. Spark retard 1126 is maintainedat a higher level until the time of marker 1108.

At time marker 1108, the amount of available reformate 1120 is onceagain greater than the first threshold 1118. This condition allows sparkretard 1126 to be reduced. Further, the injection of gaseous fuel 1122resumes in response to the amount of available reformate 1120 increasingto a level greater than first threshold 1118. The engine controllersimultaneously increases injection of gaseous fuel injection 1122 andreduces injection of liquid fuel 1124 to the engine. The liquid fuel maybe reduced at a rate that corresponds to the rate that gaseous fuelfills the intake manifold.

At time marker 1110, the amount of available reformate 1120 hasincreased to a level greater than the second threshold level 1116. As aresult, the engine controller allows the amount of gaseous fuel injectedto the engine 1122 to increase to a level greater than the desired levelof gaseous fuel injection. In accordance with increasing injection ofgaseous fuel, the amount of liquid fuel injected 1124 is decreased.

At time marker 1112, the amount of available reformate 1120 hasdecreased to a level less than the second threshold level 1116. Theengine controller responds by decreasing the rate of gaseous fuelinjection 1122 and increasing the amount of liquid fuel injected 1124.Thus, under some engine operating conditions reformate is conservedwhile during other engine operating conditions reformate is consumed bythe engine at a rate that may be higher than desired. In this way, themethod of FIG. 10 may be used to preferentially use or conservereformate.

Thus, the method of FIG. 10 provides for operating an engine,comprising: operating a fuel reformer and producing a gaseous fuel; andlimiting a rate of injection of said gaseous fuel to at least an enginecylinder in response to an amount of gaseous fuel in a storage tank lessthan a threshold amount when said storage tank is not empty. Further,the gaseous fuel is comprised of vaporized alcohol or H₂, CO, and CH₄.Further, limiting injection includes stopping injection. Further, thelimiting injection includes injecting gaseous fuel at a rate less than adesired rate. Further, the limiting injection includes injecting gaseousfuel at a rate less than a desired rate, and said limiting injectionfurther decreasing gaseous fuel injection as said amount of gaseous fuelin said storage tank decreases. Further, the method includes injectingan amount of liquid fuel to said engine. Further, the method includesadjusting a cylinder charge varying actuator in response to saidlimiting injection of said gaseous fuel. Further, the method includesoverriding said limiting of said injection of said gaseous fuel duringpredetermined conditions. Further, the method includes operating saidengine by solely injecting said liquid fuel after said predeterminedconditions are no longer present.

The method of FIG. 10 also provides for operating an engine, comprising:operating a fuel reformer and producing a gaseous fuel; limiting a rateof injection of said gaseous fuel to said engine when an amount of saidgaseous fuel stored is less than a first amount; and increasing anamount of gaseous fuel injected to said engine and decreasing an amountof a second fuel injected to said engine when an amount of gaseous fuelstored is greater than a second amount. Further, the gaseous fuel iscomprised of vaporized alcohol or H₂, CO, and CH₄. Further, the methodcomprising injecting an amount of liquid fuel to said engine. Further,the method comprising reducing injection of said gaseous fuel as saidamount of gaseous fuel in said storage tank approaches said thresholdamount. Further, the threshold rate increases as said amount of gaseousfuel in said storage tank increases. Further, the threshold ratedecreases as said amount of gaseous fuel in said storage tank decreases.Further, the amount of gaseous fuel injected to said engine when saidamount of gaseous fuel stored is greater than a second amount is limitedto an amount of gaseous fuel that forms a substantially stoichiometricmixture in a cylinder of said engine.

The method of FIG. 10 also provides for operating an engine, comprising:operating a fuel reformer and producing a gaseous fuel; limiting anamount of said gaseous fuel injected to said engine when a stored amountof said gaseous fuel stored is less than a first amount; and increasingan amount of gaseous fuel injected to said engine and decreasing anamount of a second fuel injected to said engine when an amount ofgaseous fuel stored is comprised of more than a threshold amount of afirst gaseous fuel. Further, the first gaseous fuel is vaporizedethanol. Further, the method comprising, stopping injection of saidgaseous fuel when a desired rate of injection of said gaseous fuel isgreater than a threshold. Further, the gaseous fuel is comprised of asecond fuel, said second fuel comprising H₂, CO, and CH₄.

As will be appreciated by one of ordinary skill in the art, routinesdescribed in FIGS. 2, 3, 6, 8, and 10 may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various steps orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the objects, features, andadvantages described herein, but is provided for ease of illustrationand description. Although not explicitly illustrated, one of ordinaryskill in the art will recognize that one or more of the illustratedsteps or functions may be repeatedly performed depending on theparticular strategy being used.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

1. A method for operating an engine, comprising: operating a fuel reformer and producing a gaseous fuel; and limiting a rate of injection of said gaseous fuel to at least an engine cylinder in response to an amount of gaseous fuel in a storage tank less than a threshold amount when said storage tank is not empty.
 2. The method of claim 1 wherein said gaseous fuel is comprised of vaporized alcohol or H₂, CO, and CH₄.
 3. The method of claim 1 wherein said limiting injection includes stopping injection.
 4. The method of claim 1 wherein said limiting injection includes injecting gaseous fuel at a rate less than a desired rate.
 5. The method of claim 1 wherein said limiting injection includes injecting gaseous fuel at a rate less than a desired rate, and said limiting injection further decreasing gaseous fuel injection as said amount of gaseous fuel in said storage tank decreases.
 6. The method of claim 1 further comprising injecting an amount of liquid fuel to said engine.
 7. The method of claim 1 further comprising adjusting a cylinder charge varying actuator in response to said limiting injection of said gaseous fuel.
 8. The method of claim 1 further comprising overriding said limiting of said injection of said gaseous fuel during predetermined conditions.
 9. The method of claim 8 further comprising operating said engine by solely injecting said liquid fuel after said predetermined conditions are no longer present.
 10. A method for operating an engine, comprising: operating a fuel reformer and producing a gaseous fuel; limiting a rate of injection of said gaseous fuel to said engine when an amount of said gaseous fuel stored is less than a first amount; and increasing an amount of gaseous fuel injected to said engine and decreasing an amount of a second fuel injected to said engine when an amount of gaseous fuel stored is greater than a second amount.
 11. The method of claim 10 wherein said gaseous fuel is comprised of vaporized alcohol or H₂, CO, and CH₄.
 12. The method of claim 10 further comprising injecting an amount of liquid fuel to said engine.
 13. The method of claim 10 further comprising reducing injection of said gaseous fuel as said amount of gaseous fuel in said storage tank approaches said threshold amount.
 14. The method of claim 10 wherein said threshold rate increases as said amount of gaseous fuel in said storage tank increases.
 15. The method of claim 10 wherein said threshold rate decreases as said amount of gaseous fuel in said storage tank decreases.
 16. The method of claim 10 wherein said amount of gaseous fuel injected to said engine when said amount of gaseous fuel stored is greater than a second amount is limited to an amount of gaseous fuel that forms a substantially stoichiometric mixture in a cylinder of said engine.
 17. A method for operating an engine, comprising: operating a fuel reformer and producing a gaseous fuel; limiting an amount of said gaseous fuel injected to said engine when a stored amount of said gaseous fuel stored is less than a first amount; and increasing an amount of gaseous fuel injected to said engine and decreasing an amount of a second fuel injected to said engine when an amount of gaseous fuel stored is comprised of more than a threshold amount of a first gaseous fuel.
 18. The method of claim 17 wherein said first gaseous fuel is vaporized ethanol.
 19. The method of claim 17 further comprising stopping injection of said gaseous fuel when a desired rate of injection of said gaseous fuel is greater than a threshold.
 20. The method of claim 18 wherein said gaseous fuel is comprised of a second fuel, said second fuel comprising H₂, CO, and CH₄. 